TS 90 MX is a TS 100 rolling frame very different to a TS90 , look at pics in previous post TS 90 MX has a single down tube frame ala TS 100.
TS 90 MX motor is very similar to a TS 100 with different Aluminium cylinder , rotary disc, manifold , 28mm carb, Internal rotor CDI ignition, expansion chamber .
The Ignition and chamber are very similar to a TM 125 but are not interchangable,
TS 90 MX is very different to a Kitted TS 90
and shares its origins with the TS 100.
Brent J knows a lot more about them than i do , he had one in the day. I believe they were sleeved down to a 90cc to fit into Japanese domestic race classes , much the same as a TS 185 hop up kit is actually 175cc to fit in the 175 race class