You just made me realise something Doc. I could never work out why the TS90MX was called as such and why they were 90cc when they were based on a TS100. Your comment about the Japanese market getting the TS90 up til ‘76 made it all clear.
Our TS90 was superseded by the TS100. While it shared a lot of parts with the TS90 there were some major differences. The frame, pipe and tank are certainly different and the 100 has a longer wheelbase. I think some of the gearbox internals will interchange but the cases and cylinder are different, the cyl stud spacing is wider.
The second Japanese TS90 was a de-bored version of our TS100.
For what ever reason, tax maybe?, the Japanese stuck with 90cc while we got the 100cc version. The MX90 is 90cc because it was a JDM model and based on THEIR later TS90, if it was exported it probably would have been a TS100MX.
And to make this post relvant, I don't think I've ever seen an aftermarket head for a TS90 but I do wonder if the factory hop up kit had a different head?
Brent