Engine options?

Bikeberry says on the 66cc whatever it is "The frame should have 9-11” of clearance between the bottom bracket and top bar, and the diameter of the frame tubes should be 25-29mm (around an 1”). 4 stroke says "48cc engine kit is about 9" height 9.5" length 7" width. The bicycle gas engine kit includes a mounting plate that fits on standard 25mm to 29mm frame tube. Larger frame size requires modification to the mounting plate and/or frame of your bicycle.

Flying Horse 5G 4-Stroke bicycle engine kit. Your probably going to have to find out for yourself if it will fit or not. Or you could get an electric kit.

A guess from looking at the attached pic, rack mount. Judging at eyeball, from the geometry of that frame, nope, nopeity nope. Too much pinch, yah the measurements are ok, but not relevant with the angles present in both top and bottom bars. Look at the bends...

Rack mount, be it friction, belt, or chain drive, don't care a lick about what's ahead of it. I'd imagine an intrepid (read: insipid) individual could easily motorise a unicycle with what's out there now a days. Out side the odd, non conventional bikes, this industry has met and defeated most challenges. Fact is, surprised as I am, bikes are not just for pedaling...in reality that's for the ambitious...

Rack mount dude, it's the easy answer, at most you need a pair of frame stays... About a twenty count with a welder... Grandpa would have charged you lunch, maybe $8? And he'd want to know the story...

It would be very difficult to do a center mount engine on that bike, it just looks too small to me.

Welcome,
Use that as a pedal bike and build a different bike.