I had a chance at a 600cc Rotax (110hp), that I wanted to put on a trike, but unfortunately it sold a couple days before I could get the money together. I imagine people have put some rather large engines on trikes though, or even those little push trailers. Otherwise, the largest I've heard of was 212cc. Though I think it'd be ridiculous and funny for someone to mount one of those 5cc nitro engines on a bike, you know, the ones that crank insane amounts of power for their size?
Another concept I wanted to do after I got on the oil rigs was buying a pair of model jet engines ($3000, 25 lbs of thrust each run on Kerosene, IIRC 250mph top speed) and run the HT engine for the low-mid speeds (city travel) and the jets take over at high speeds (and maybe a 3-way hybrid with electric for accelerating). A bit of a pipe dream. But some people HAVE installed a pulse jet (imagine a really, really big 2-stroke expansion chamber, but the "motor" IS the pipe) on bicycles and hit around 110-120mph with them.