Jason,
I fully understand your dilema. I'm in a similar situation as you. I love my 250 but want to try out the 300's power characteristics. I was planning to keep my stock parts too just in case I didn't like the 300. I have been pricing a full conversion and it's a little steep ($$$). To do it right, you will need the cylinder, head, piston kit, CDI, and change the spacer on the reed valve. At this point, I'm on the fence about the conversion. Not sure in the investment can be justified since I would never recover it when I eventually sell my bike.