Streamliners are open and only limited by engine size, fuel and if it's blown. Skys the limit!
Well, yes and no. For SCTA you run in a class structure. There is a frame class and an engine class. If you are running a streamliner it must be a motorcycle engine with one exception. You could even run S-P with a Production engine.
G (gas), F (fuel), BG (blown), or BF state:
Unlimited in design, but must be comprised of major parts and components designed primarily for use in motorcycles.
Production, pushrod, and vintage and all the variations must be a motorcycle engine.
The exception is UG and UF. It has to be an Otto cycle and run in streamliner only. I don't think anyone has ever run this engine class.
On the other hand, I don't know what else the BUB liner could be.