Doesn't the front sprocket move further out on the TCat.
The TCat / 4JH motor is fine in a 3TJ frame but if all you want is a good quick job go for the 3he into a 1WG frame.

mmm I seem to remember I had a sprocket carrier turned down a few mm for a project..but cant remember if it was for the T'cat bike. It's not a big job though for an engineer.

I was trying to keep it easy, cheaper too

The 3HE dosen't have a problem with the sprocket.