Suzuki VanVan, saw shitloads in Japan, great little fun bikes, comfy for short journeys too. Autotrader - SUZUKI RV125 VAN VAN 125cc
Honda NSR 125, similar in speedy speed as the aprilla but bit better reliability. I have a chinese pit bike and they do love to break, but ive only ever done the chain in on mine. Was going to road register it for commuting to work etc,
i wouldnt go for that mountain bike engine kit, i think you would need to get an SVA test then get tax and insurance to make it road legal.
i have a tzr125 streetfighter with full powervalve system which is good for going to work and back and generally messing about on and is good for 100mph . but my new 150cc pit bike is alot more fun as it gets a back wheel in 3rd . i aslo have 3 chineese pit bikes which i havent had a problem with apart from snapping chains in big jumps . and a 125 thumpstar which just eats back wheels .