2007 RM250 Gearbox Modification

Hi, Here's my first post ! 

So I have a 2007 RM250 & have been made aware that they are known for having problems with the gearbox. I have read that there is a modification that can be done to extend the life of the gearbox in these bikes. 

Here is a copy & paste from another member of this site explaining the mod. Only problem is what size shim washer & better circlip do I use ? 

"- 3rd and 4th drive gear and 5th driven tend to wear out on the dogs about every 60 hours running time (on average)Theres a shim washer and circlip on the back of 3rd drive gear thats a little too thin and allows 3rd to 'float' a little too much. We add a thicker shim washer there, and also a more heavy duty circlip- the stock circlip can concave (or pop off all together) if you stomp on the gear lever too hard, allowing the gear to move where it shouldnt. This will make your gearbox last alot longer!"