I've used a few different gas-powered pumps on an ATV trailer sprayer, which is just a slightly larger version of what you're trying to do.  Two versions used a Hypro Big Twin piston pump powered with either a B&S or Honda 6:1 gear-reduced horizontal shaft motor with lovejoy coupler.  The current, safest and simplest model has a Honda motor mated to a 3600rpm Ace centrifugal pump.  100gal tank, so the 22ish gpm pump is waaay too fast wound out (foams up like crazy with some chems), but works well down on near idle.  Compact enough for your UTV bed I'd think.  You probably can't tell from the small pictures of mine that there's a garden hose fitting above the blue surge tank if I want to hook up a hose with hand nozzle of some sort.  I never liked spraying with dye (couldn't see it).   I bet you could find a smallish used foamer around somewhere for cheap though.

http://talk.newagtalk.com/forums/thread-view.asp?tid=84892&posts=12&highlight=hypro%20piston&highlightmode=1#M615329

http://talk.newagtalk.com/forums/thread-view.asp?tid=86078&posts=4&highlight=hypro%20piston&highlightmode=1#M615898