Once again it seems like someone is trying to diagnose by guesswork?
Basing decisions on hunches are invariably influenced by whatever your particular prejudice or belief is, or what the 'bloke in the pub' said it was. A good way to waste money on unnecessary parts and time.
There is no secret to good diagnosis - base your decisions on as many FACTS that can be established.
By all means ask for opinions and draw on others experience. Whilst this will be offered in good faith, remember that they haven't seen you bike misbehaving.
Take on board what they say then test the advice by measuring or examination.
If still unsure ask again then re measure or examine the part.
Sorry to start with a rant but I hate to see folk wasting time and money.
To your problem.
Can we assume that the carbs are balanced, fuel filter is clean and has been replaced within living memory, there is no rust in the fuel tank and it idles when up to operating temperature?
You say the motor is overheating. How do you know this?
The temperature indicator will be around halfway on the scale at normal operating temperature and just below the red before the fan cuts in.
From the way you describe the problem it would seem this is happening with the temp. indicator sitting at halfway?
If this is correct then this is not an overheating problem but before discounting it check that the indicator is operating correctly (see the Service Manual for how to do this).
Lets assume that there is a problem then IF it is the thermostat you can measure the temperature at which it opens.
IF it is faulty you replace it if it isn't then save your money.
If it's not that then move on to the next most likely candidate which would be a partial or full blockage somewhere in the core or the radiator. Yours is some 28 years old so after that time there could well be issues.
Is the core clogged with dead flies or mud; are the fins all crushed? Anything that impairs airflow will reduce the efficiency.
As the motor heats up feel for cold spots in the core. It's probably best to remove the stone guard to do this. I've not tried it but an infra-red thermometer may also work.
Try radiator flushing chemicals and reverse flushing. Less likely candidates are cooling fan not cutting in but this should only be an issue when stationary, blown head gasket but I would expect to see coolant being pushed out of the overflow or impellor spinning on the water pump shaft. Hens teeth are more common.
One other issue that I have (rarely) encountered in traffic with high ambient temperatures is fuel vaporisation. This cleared once I was on the open road. Possibly more likely if the cylinder head heat shields have been left off.
If it is voltage related then this can be easily checked with a voltmeter e.g. the R/R whose output can be measured.
Once again, IF it is faulty then replace and save your money if it isn't.
As I said above, the way to ensure a first time fix is to base all of your decisions on the best FACTS that you have available.
You may strike lucky by guessing or going on a hunch but invariably you will waste a lot of time an be poorer.
Your choice.:ummm: