That's an MX100A or B. Basically the same bike as an MX125A, but with a smaller front wheel and steel lower fork legs.
Confusingly, the later red MX100s that came with an early IT-style headlight (and RT100s) were actually more closely related to the earlier LT2....
The two obvious differences are the single backbone frame and leg burner pipe on the LT and later MX/RT.
Standard wheel sizes on a MX100A are 18 or 19" front, with an 18" rear, IIRC.
It looks like this bike has a front end off a twin shock DT175 or MX125 - 21" front wheel and slightly less wimpy forks.
Cool as.
