Can you explain a little more about needing 2 right hand cranks. Is that to allow the pedals to still operate as usual? I may have been over excited I'm my first response and not understood completely.
Overall, I defiantly still want the pedals to operate normally, which may be the biggest problem.
5-7 I wonder if found someone to re-thread a right side freewheel so it will fit the left crank if that would work?
You need 2 right hand cranks because the rh one has the threads to mount the freewheels. The left one does not. It simply slips onto the square shaft.
They make cheap, low-quality left-side Chinese pocket bike freewheels. Unsure if there's high-quality White Industries left freewheels. Then you'd have to machine the right arm to accept left-hand threads.
Once, I installed my right-side freewheel backwards on the crank arm. Then it acted like a left-hand freewheel. It is EXTREMELY difficult to remove the freewheel from the crank arm. Locktite and pinning the freewheel to the arm would lock it in good to work on the left side. The engine would always be trying to loosen/unthread the freewheel, so you need to prevent that.
The left-side crank MIGHT have to be extended further out. Also unsure if the engine and chainring sprockets would line up.
Also unsure if the chain would run from engine to chainring, even with the cover removed.
If everything aligns, then a 10-tooth engine sprocket and 48t chainring would be 4.8:1 x 4.1 engine = 19.68. A standard shift kit has gearing of (4.1 x 1.7) x (44t/10t) = 30.67:1, BEFORE connecting to the rear cassette. By the time it got to the 32t 1st gear on an 8-speed cassette, the weakest combination of a 30t chainring and a 32 1st gear would be 29.44:1.
If you could fit a 9t on your engine and use a 52t on the left chainring, that's be 23.68:1.
Then with a 24t right-side bicycle chainring and a 34t 1st gear on the rear cassette, you'd be okay with 33.56:1. Using a 22t chainring would give you 36.61:1 gearing.
Eighth gear would be 11.84:1, comparable to a 29-tooth rear wheel single sprocket on a Happy Time install.
It is always advisable to gear your bike as low as possible. In doing so, you get a granny gear for very steep hills. You also get to use 7th or 8th gear as a highway cruising gear. I use a Tanaka engine with a shift kit. My 1st gear is 46.36:1, which I find VERY handy. My 8th gear is 15:1, which is also very usable.