RE: KLX 300 Cylinder
There's no doubt in my mind that both will fit. The PN for everthing elses is the same(gaskets, alignment pins, piston, rings). They may havechangedthe material used for the 'electrofusion' lining or some other thing that has no bearing on fit.
The only problem you would run into with fit is if you had a 1997 or prior KLX250 where the case opening is too small.
+1 on going with the newer one though. You might want to visit the thumper talk KLX/KLR forum and ask them specifically what changed. There are some pretty knowledgeabledudes over there when it comes to the KLX300.
Yamaha XT1200Z Super Tenere
Previous Ride: D-tracker 300cc, Wiseco piston, Yoshimura pumper carb, Nassert stage 1 cams, Moto-pro suspenders.