I need help selecting engine!

I just bought an adult sized Schwinn green chopper and plan to install an engine but having a tough time on deciding what kind. I have narrowed the choices to 4, the Grubee skyhawk, the 80cc slant head limited silver edition, wait for sickbikeparts new inframe 4 stroke or the Dax new in frame 4 stroke thats supposed to be soon available. The Schwinn chopper has the fat 4" wide rear 20" tire, also wondering what size sprocket will propel my 250+ lbs up moderate hills, am thinking 44t? I'm pretty excited about this build, there is some spoiler springer forks on eBay for around 80.00, wondering if will fit on this style of chopper? Recently sold my Geno cruiser with a 33cc rear mount, the jury is still out on last winters build, a Schwinn point beach ladies cruiser with a Dax 4 stroke rear mount, springer fork and clam shell sprocket adapter.
Any ideas or thoughts sure will be appreciated.

Seanik