Alrighty man, weird stuff. You must have Alzheimer's because you've made a thread on this topic already:
https://www.newschoolers.com/ns/forums/readthread/thread_id/730222/
Next time I'd bump my original thread instead of making a new one if I were you. Anyways, here is a copy/paste of my last reply in your last thread, some good resources in there.
This article here should give you a really good idea of what it would take to build one:
http://www.sbcskier.com/features_article?news_id=836
Was written by Logan Imlach. His NS profile, I'm sure he'd be down to help ya out with a few questions (Sorry for putting you on the spot Logan)
https://www.newschoolers.com/ns/members/profile/member_id/22517/
Good luck man. Post pics if you make it!
And to answer this threads questions.
1. A centrifugal clutch would probably work best.
2. A 1" axle is way easier to find pillow blocks, gears, and other hardware and parts for as opposed to a 3/4" shaft. It's also stronger and that never hurts.