If I remember correctly the 300 was kickstart, whereas the 250 has electric start, unless I am mistaken. Goodnews is you can bung on the 300cc cylinder and piston as its a straight swap. Bolts on no problem. The cylinder head ,valves, cams etc are the same for both bikes.
Step two would be a flatslide carb. 33mm for low end grunt and 36mm for some topend. The stock CVK gets great gas mileage but is as slow responding as a hungover bar girl. These will cost you about 7000 baht in Thailand used. They needed to modded a bit to bolt on inside the frame. Then bung on the 300cc cylinder. In the states you can bore the 300 cylinder out and re-Nikasiled to 331. This would result in a nice torquey electric start bike for fairly cheap.
When dialing in the carbs sometimes jets are not easy to find for your carbs in Thailand. Nice to have a friend bring a supply over for you. Dynojet makes a kit for the stock CVK carb as an easier option.
I only know of two companies that make cams for the bike. Yoshimura in Japan, expensive at about $700 or WebCam in California that regrinds your stock cams for $275.
Do the stage 1 mods I mentioned. The bike is more of a wheezer from the factory than any other bike I've ridden. Plugged up worse than a katoey in an orgy. It really benefits to have the airbox and exhaust free'd up to do their job.
Good luck.