What Honda does sometimes is they "supersede" part numbers on "older" bikes to a new number , but list it as "part no longer available" when its actually the same part from year to year , but the part number is actually what is no longer available and only different because of the year change , but physically its the same part , like i mentioned , a "few" parts are physically different from year to year from 02-04 , but mostly the parts interchange and are the same , because they find things that work better or are cheaper to manufacture from year to year , and alter the bike as such
The new 450 R/X , they changed the case to omit the kickstart like the way KTM did over the last few years , its a simple change, and instead of a case with a plug where the kick start was , or having a E-start and a kick start , its a case without the hole that needed a plug when the kick start was removed , its more cost effective for Honda and shaves a little weight off the engine due to less material because the raised and reinforced portion of the case is now removed , the downside is , you can no longer add the kickstart like the year prior without replacing both inner and outer cases , but it still bolts to the engine exactly the same for both parts , and the rest of the engine and bike remained unchanged , with the exception of header length
Personally i think you will be fine , and if your buying the case "new" and don't actually install it , then i am sure you could return it (but don't tell them its for a different year than stated) , or , as mentioned , search eBay for a same year replacement (used) and your good to go and at a far lesser cost ...