Hi.
i dont know anything about the 300's,but on my 200 twinshock i sent my barrel and head of to jetbore,they matched it with a suzuki TS175 Piston,but be aware of matche's when it come's back to you you might have to cut the piston up abit and make a thicker base gasket and the barrel wont be chromed, but bike run's perfect with the conversion on the 200.
you might be lucky and someone on here might have the origanal part's your looking for?
jetbore..01902 739650
it's worth giving them a call,they knew what they was talking about with my 200.
cheer's