Jus' a thought...
If you're going through the effort of custom fabricating a frame to facilitate a "longer... and altogether lower design" ...have you considered a new/used moped engine? They're of a much higher quality than the Chinese kit engines w/a plenitude of groovy aftermarket parts:
treatland - they're not common on motorized bicycles due to they fact they're horizontal, which in this case may not be a problem...
Ha, nice one - I've been looking at them this week too - here a China girl kit is near enough to $300, for the same money you can pick up a pit bike new, or a better quality one second hand for maybe even less. Wrecked scooters run at about $450ish, and you get all the lighting with it too - bonus. And you don't need pedals to register as moped, just under 50cc and 2kW output.
Thing is these bikes have small wheels, but that can be worked around. This comming weekend I'm picking up a Suzuki TS50 (Gaucho), I got it for $50, tank and seat are busted, and apparently the motor smokes (funny being 2 stroke). The point of that exercise yields me 21" wheels, tripple tree & forks, maybe a wiring loom, and fun stuff to re-purpose.
Still - having said all of that, I'm dedicated to doing likewise to OP and going horizontal with one of the kits I already have spare - WHY?
Well, why not - I mean, why would you spend the winter months mucking about with trying to reinvent the wheel anyway... Remember, the key to all forms of rocking is style!