Before buying any old thermostat that fits the hole here are a few things to consider.
The factory thermostat is an offset bypass type which actually controls the water to the radiator and the bypass. Thermostats without the bypass valve will alway allow water through the bypass regardless of engine temperature and thermostat open or closed. The jiggle pin is there to allow air to escape the system when filling.
My original was replaced years ago by a TCI thermostat and is still working fine. I've made some measurements of it:
Flange diameter = 44MM or 1.733"
Bypass valve diameter = 28MM or 1.107"
From base of flange to contact face of bypass valve = 37MM or 1.456"
It is rated at 82 degree C or 180 F
I've found a few that are close but as yet no cigars. One of the things I'm missing is the depth of the housing to the bypass hole and the diameter of the bypass hole itself. Because the bypass valve is spring loaded there may be some give in the valve distance from the flange. Also, depending on the hole diameter something smaller may work. This just reminded me, I also need the diameter of the bypass chamber in case a larger valve may fit.
If someone can provide these numbers I'll see if I can locate a suitable part. I'm also going to try contacting MotoRad who was supplying the part for TCI. The part number on the box does not come up when searching their site. Searching is also slow because a lot of time dimensions are not provided and deeper searching is required as well as cross referencing.
FYI - The factory O-ring is listed as 43.5 x 2