[keefer]

if I were getting a small track bike I would go for the two stroke.
its lighter more tuneable and easy to work on. and I would imagine depending what model you go for parts are more readily available.
I like the mini fours. but if were looking at cheep easy track fun, two is the way for me
best bet is a RGV M loads about. the aprilia is just a Suzuki engine in there frame. Yamaha make good track bikes in the TZ range. i wouldn't bother with any of the road going versions on track. (to fragile)
or you could just get a Kawasaki KR1S if tank slappers are what your after