This rotary valve two stroke is one of the toughest units on the market. Fifty quid buys you a 1975 to '77 model. Not very fast at 60mph when it's worn out, not particularly economical at 70mpg, but the frame, suspension and cycle parts are all jolly good stuff, way ahead in the quality stakes when compared with other single cylinder commuters.
The engine buzzes along flat out for ever and the chassis doesn't hold any nasty surprises - even the SLS drum front works alright. It's possible to increase the size of the holes in the inlet disc to obtain some more power. I know someone who managed to get 75mph out of one, although low speed running was a little erratic and after six months the engine disintegrated in a rather loud manner...
There are still some around in surprisingly good condition, but don't expect too much for this kind of money, you'll need to spend some time and money putting things in order, but the KH100 is a good basis for renovation.
There aren't many in breakers, but at least Kawasaki spares are some of the cheapest around and the engine has one of the few attributes of two stroke technology - it's easy to work on and needs no special tools except for a reasonably large hammer.