Post looks to be getting stale but i try to help a little. I know a guy who has a lot of work done to his. If he says it i am inclined to believe. I had asked him one day about it, I have a trx300fw. We were on a ride and didnt go into detail. Yes the top sure did not look like my stock FW. It was missing the compression release. Said you need the head and jug from the EX. I would guess internals too cam, rockers, etc. Maybe we are lucky and the exhaust will bolt up. I know you had said in another post the two engines spin different ways. I am sure you can find a friend with a 300Ex to find out for sure.
2000 TRX400EX:
Open Air box
Uni Filter, choke removed
Port work and valve job
Cam
426 Bore High comp piston
HMF Exhaust
Pro Armor Fat Peg Nerf Bars
Why did i sell it?
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1998 TRX300FW
27 inch Swamp Witch's
2000 pound winch
Uni Filter
Supertrapp Exhaust
XP ATV front disk brakes
Warn 424