That looks like it would work well, but pricy. I scrounge
most of my parts from a local bike boneyard, i.e., the
Recyclery. Still, if you can't find it anywhere else, Niagara
will probably have it and they're good about getting it out
quickly.
I don't know how heavy you are, Cuz, but with a 49cc
or less engine, I'd say you could get away with a 16" rim
using a 1" primary pulley because the 20" wheel already
give a lower ratio than a 26" wheel. Kid's bikes go dirt
cheap at garage sales, often for less than the price of
a new 16" tire. The 1" pulley can be found on line with
a bit of search. It might be worth it to try it.
V Belt Supply has belts in any length you'd want for
around $5 + postage. A 3L or 3V is what you'll need.
both will stretch a bit. the 3V gives more contact but,
being stiffer they require more breakin and don't perform
well until they are warmed up a bit.
You have to allow for some stretching adjustment to tighten
the belt. Their site has good instructions for measuring belt
size.
Get your 1" pulley first to be sure it will match your choice
of belt.