I am glad you are going to a steel frame.
Vibration is a big issue on these engines. I have a hf 212 on bike 2 and it vibrated seriously. Ordered a Dupor and put it on instead and it vibed just as bad. Put it back in the box (spare motor?) and isolated the HF with rubber mounts.
I suspect vibes are directly in proportion to piston size.
The gokart stock class clones have no issue with the flywheels as long as the valve springs remain stock (this limits rpm to around 5000- for more info go to www.karting.4cycle.com
I spent a lot of time there). I think the better flywheel would be a good idea though, sure would feel safer.
The better rod is needed for high rpm, an item you don't need if you leave the cam and springs alone. A re-jetted stock clone is good for 9+ hp when it can breathe properly, meaning a free flowing exhaust and intake. Lots of torque stock.
Gas milage has been real good because the engine is loafing most of the time.
I really prefer to use the governor, with it screwed as high as it will go (about 4 grand) but the Dupor has no provision for one.
These are sweet motors as far as reliability goes, however, the gokart people take them apart to make sure they are assembled right and assume this to be standard proceedure.