i would check your local lawm mower shop cause i did in my old town and got about 4 5.5hondas and a few old briggs flathead for about $150 SOME NEEDED WORK BUT ALL RAN
Try craigslist, i picked up an 8 and 3.5 horse for free the other day, the 8 doesnt have spark but the 3.5 runs. i also seen tons of engines or machines with good kart engines for under 100.