Nice bike! Man there's something about them horizontal Hondas! BEST fuckin motors ever built!
In my opinion still, the most simple, bullet proof motor. I have a 2007 CRF 50 with that motor with a smaller bore and 3 gears, and it is a hell of a motor. No frills, but simple, bullet proof reliability. I never even change the oil, doing it this weekend. And it just runs like a dream.
I have put that 50 thru a lot! Me and my big fat friend road it 2-up one time! Totalling about 400 lbs! And its rated for 88 lbs! Huh!
Still pulled well, in 1st and 2nd still plenty of power! Road it down the dried river under the ranch [almost an obstical coarse], almost flooded it thru a huge puddle, gone mudding all over the ranch and flew thru piles of manure, ate shit plenty of times [not literally], etc. But its ALWAYS, since day 1 started on the 1st kick, rather it set for a year or a day, cold or warm. Doesn't even need choke! But one hell of a bike. Not my only dirtbike. But a blast! Perfect for screwing around at the ranch, campgrounds, etc! Not good for long trail rides stock, but perfect for screwing around and wheelies! And I hit 40-45 MPH no problem!
Anyways, nice bike you got! Looks like a fun project! I got a 90 cc Mini Trail chopper project street legal too. Its not like that one tho like a mini bike originally a 50 or 70.
But if I was you, I'd stick with that motor, for it's 3rd world country design, unless you really need extra cc's. Lifan motors are great too. A 140 cc would give that some power. Also 160 cc some brand I don't know I think it's fit that there bike with a lot of power and oil cooler. But not a Honda still. That motor as you probly know is 45 MPH, with a lot of MPG.
And Wow! A 66? Shit, 46 years old? Thats a classic! Give her some love and put some miles on her! Good luck and I'll be watching!