I agree with Mark and Andy. It is easy to find parts & tuning help for OTK chassis and ROK engines since many FL tracks support their products which is important if you have any on track incidents. As far as Rotax, x30, or ROK I would lean heavily towards ROK. The ROK engine and Bridgestone/MG tires really work well together. Plus, there are none of the Rotax EVO issues or feathering of the carb. Just press the gas & drive! I would wait a while before buying a Rotax EVO engine so they can iron out all of the issues. I haven’t driven the x30 engine but it seems to be used more out west & in SKUSA. If you decide to go the OTK chassis route I am getting ready to list my slightly used 2014 30mm chassis with new stickers/plastics at a good price. Best of luck with whatever you choose.