I'm with George on friction being easiest to build and use. Plus, if you just
want to pedal or coast, it's simple to just lift the roller off the tire. Kit-wise,
it's a tossup twixt Dax and Staton. I'd say Staton is bit more precisely made,
but Dax offers more hardware and is more adaptable. Which ever you choose,
I'd buy the engine separately. Don't get the smallest roller no matter how
heavy you are. If you're 200# or less, a 4 stroke honda gx35 is super quiet
and super reliable for $240 at smallengineswarehouse.com. If you don't
want to spend that much there's a variety of cheaper 2 strokes out there.
you just want to be sure their clutch will match your kit. Monster Scooter
Parts sells a very strong 49cc 2 stroke for About $130. There are also
many of the same type available on Ebay. If I had the dough, I'd look
for a Tanaka PF4000 2 stroke,;it's virtually as clean and quiet as a 4
with relatively more power.