Re: bmx w/Yamaha 100
you could also check craigslist for junk mopeds or ebay for 16" moped wheels.
as stated above, a 16" moped wheel is the same as a 20" bike wheel. it's the only size that wll cross over. there's no compatible moped/motorcycle tire that'll fit 24 or 26" bikes.
mopeds use thicker spokes and bigger hubs with bigger axles and are designed for faster, heavier bikes, and aren't much wider than a bicycle wheel.
plus you can use the big drum brake hubs for better stopping power than a crappy bike hub.