My suggestion is to evaluate what gear your riding in mostly on said track , as MX in general has a wide variety of track lengths depending on the track design and location , for example , most of the tracks where i live are short (i hate that) and tight , which to me is NOT a real MX track , where i "normally" ride , which is a clapped out unmaintained location , i ride on a extremely rough , fast sketchy layout where i actually (mine is a 02 , but gearing would be similar) geared up to 13/48 , where stock was 13/50 , i could actually go to 13/47 and be fine because of the higher speeds , that said , i try to gear my bike according to the conditions i actually ride (which means where you ride and where i ride would most likely require different gearing , so there is no "specific" gearing for MX that fits all)
The goal is to be able to ride the track or tracks you frequent , without constantly shifting , so you may be fine with stock gearing , or you may need 1 or 2 teeth up or down to get you to where you're not constantly shifting , only riding the tracks and paying attention to your shifting and what gear you use the most is going to determine what actual sprocket combo you need
What i would do is figure when your riding , what gear your in the most to have response and the ability to do the majority of the obstacles , if you are having to shift up and down constantly for turns and straights and can't hold a single gear for a large portion of the track , then you need to go up or down accordingly to allow you to stay in 1 gear longer throughout the majority of the track , shifting down 1 gear for corners is fine , if your going from 4th to 2nd to make a corner , then your gearing is not right for that track or your speed/ability , for a example , in SX , the "pros" usually only have 2nd 3rd and 4th gears in their tranny , for weight savings , they choose sprocket combos to suit each tracks obstacles and speed to allow them to stay in 2nd and 3rd for the majority of the track , so they are only having to shift once in a while , as the less shifting your doing , the more you can concentrate on actually riding the track with flow and less likely to "miss" a shift which could cause serious issues when jumping , so for MX , its the same just on a larger scale , what you do is ride the track , trying to keep flowing and pay attention to what gear your in mostly , if your constantly shifting from 4th then having to shift quickly back to 2nd to make a corner , then you need to gear your bike down so you can hold either 4th or 3rd longer eliminating the majority of shifting to maintain pace and the need to drop all the way to 2nd ot get around the corner without stalling or over revving , if your shifting to 4th because your winding the RPM's constantly trying to hold 3rd and its holding you back , then you need to gear up , the goal is to gear so you don't spend the majority of your time shifting , so you can hold only shifting 1 or 2 gears up/down for the majority of the track
1 tooth on the front is roundabout equal to 3 on the rear , , but usually 1 or 2 teeth would remedy your issues , so the rear is the one to change unless your really having to shift thru the gears and your either way too low or way too high geared for the track conditions , accordingly , a larger rear sprocket would likely require a longer chain (more links) , also chain length will affect handling so keep that in mind when swapping sprockets , the farther back the tire , the slower turning the bike will be , the farther forward , the quicker steering will be due to the change in wheelbase , this will also affect weight bias when jumping , the shorter wheelbase would be more responsive to throttle input and rock the bike back or forward according to throttle input easier than the longer wheelbase
With all that said , what i was trying to say is there is no "set" gearing for MX , it literally depends on the specifics of where you ride and your riding ability , a slower rider will find stock gearing fine , where a A or B rider will find the gearing maybe to low because they carry more speed everywhere , again , track conditions ruling where to go as far as up/down and how many teeth
Stock gearing is a "general" combo that suits the "average" recreational rider , with all that said (sorry for the long drawn out reply) , I prefer Non O-Ring chains due to the lesser maintenance when riding in dusty or dry conditions , as O-Ring chains tends to clog when they attract the dirt and wear faster and are a bit more maintenance , they are also quite a bit heavier if you are concerned about weight savings , they are also wider and will sometimes rub the tire if you run a 120 instead of a 110 , they also will sometimes rub the case by the countershaft if you have a front sprocket that is not offset correctly for a wider O-Ring chain , which are all reasons why i use Non O-Ring chains
But what I choose in chains is DID or ER , i have found the Renthals and other brands tend to stretch faster and just wear out faster than the DID or ER , Gold Non O-Ring chains specifically , not saying they are the best , but they have lasted the longest for me and the riding I do , I usually go with steel rear sprockets to add to longevity , as a Aluminum rear sprocket will wear 2-3 times faster than a steel equivalent .....with that said , my "opinion" is just that , a opinion , and does not mean your experience will be the same , but it has worked best for me and the riding I do.........
Hope that helped .....hopefully you did not fall asleep , while reading my long reply
