Please critique my HF 79cc 7 speed idea...

that bike is really ****ty, I had one and I returned it because I bent the rear frame just pedaling.
 
It is a cheap bike... so I am not surprised that your frame bent. Maybe this bike is not a good option.

I need a frame that is made of steel so I can weld the engine frame to the bike.

Nicer bikes are made of aluminum and I can't weld aluminum with my current rig.
 
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