Welcome to the site !!
Actually , the issue is , going to a 12 would actually cause more issues than it helps ....
First it would make the chain tighter on the pivoting end of the swingarm (closest to the engine) , which will not only bind the suspension when compressing anything past having the swingarm level , but it would also cause the chain to dig into the slider and then dig right thru to the swingarm cutting a notch in the swingarm (not good , this happened to me on my 02 450R trying to gear it for trails)
Next , it will not actually solve the issue ... what you actually need is a flywheel or flywheel weight (I put a weight on my 2020 450R , not only did it incredibly help with stalling , but it also took away a major amount of compression braking , which was making the bike lurch forwards and back when coming into or exiting a tight corner) , it is now smooth and incredibly stable in tighter turns and much more controllable in using the throttle when wanting to feather it
I would highly recommend keeping the front as is , or even going up to smooth out the power delivery , or at most raising the rear a tooth or so , but when you raise the rear , then you lose top end ... not sure how much of a concern that is for you ... so you can go 13/52 and be pretty good , but I would expect it to still stall in the tight stuff without a flywheel/weight , regardless of what gearing you go to
I have tamed my 2020 450R by adding the flywheel weight , and going 14/51 on gearing , it smooths the bike out tremendously (I mainly ride moto , but where I ride is mainly off road , as is the course I mainly ride on , I have over 90 hours on my bike and about 80 of them are with the weight and gearing changes) , but it still has plenty to wheelie out of a corner in 3rd gear when I feel the need to grab a handful .... I can pretty much idle around in deep sand and not stall the bike ... it is really amazing what a simple change did for the stability entering and exiting corners and overall ability to let off the gas and not worry about a flameout , I have not stalled in a tight corner since doing this .... if you feel you lost a bit of snap , use Map #3 , if that is not enough , get a full Yosh system , it will make it a torque monster (I have the Yosh full Titanium system on my 20) I am so happy with the performance and rideability of my bike since the changes , it is a completely different bike now .. it still has gobs of power right now , is still really really fast and quick , and still is very manageable in the tight stuff when I ride into it .....
