Can you post photo of your thermoswitch next to hole in radiator? Checking with vendor you bought radiator from might be easiest. Ask them if they have thermoswitch that'll fit.
Otherwise, you'll need to measure actual thread-size of hole in radiator.
1. Measure ID of hole with
calipers.
2. Measure thread-spacing with
thread pitch gauge
Now, once you've got measurements, there are two kinds of thermoswitch threads:
Metric thermoswitch is what bike originally came with. These tend to be 16 or 18mm with O-ring and sealed connector. If the 1st thread and deeper threads you measured are same diameter, hole in radiator is metric. Add about 1mm to your measurements to find thread-diameter of switch.
SAE/NPT threads are tapered. You can determine by measuring 1st thread having larger diameter than deeper ones. These typically come in
1/4", 3/8", 1/2" NPT tapered threads. Use teflon plumbing tape to seal when installing.
Match up specs you measured with
one of readily available sensors. If it's SAE/NPT, you'll have to chop off harness-connector and solder on 1/4" female-spade connectors. Or make adapter harness.