This machine is my friends and I have put the second set of piston and rings in it. I ask if the temp light ever went on and he says no. The piston is scored in the front and the rings are stuck. Can anybody tell me how to test the fan, coolant pump and the temp light?

Have you verified fuel flow? Did you pressure check the Engine before and after the rebuild? What premix and fuel is being used? What ratio is the premix? Was the Engine properly broken in? Did anyone due any plug readings/ chops?

Does the temp light come on when the key is turned on? It should shut off after a few seconds.

Not common for the mechanical water pump to fail. Did anyone listen for the fan to turn on or pay any attention to it? Gotta check your jetting with spark plug chops.

Did the guy just take it out and run the hell out of it and then blame you for the failure. Got a funny feeling about this one.

Do you have a manual?

I'm with rmesser on this one.
You need to do a pressure check to verify you don't have an air leak first.
My money is on a crap fuel pump that isn't putting out the correct fuel flow or even more likely that he is running shyty fuel that don't meet the minimum octane rating. You CAN'T run regular fuel.

there are 2 temp sensors on the pilot, one has 2 poles the other just one, one turns on the light the other is for the fan relay, I don't recall which is which but check them both.
with the key on pull the 2 wires on the 2 pole switch and make a jumper, a paper clip will due, jump the wires together it should either turn on the fan or turn on the light.
on the switch with the single pole, pull the wire and jump to ground, this will either turn on the light or the fan.
this will not tell you if the switch works or not but it's the first step in trouble shooting the system, if the fan doesn't work with either of the switch leads then get a wiring diagram, for the fan I believe there is a relay there may also be a fuse, one of those could be bad or a wire. if it does work then it could be the switch is bad or it just is not getting hot enough to turn on the fan.

one more thing, make sure it has coolant, the temp switches are on the radiator and it won't get hot without

The temp sensors changed between 89 and 90, so we need to know what you are working on.

Man you guys are the best! Its a 1990 FL 400. Leak down test I just did passed. He used my non ethanol fuel with a 20/1 Klotz oil mixture. I don't believe the temperature light ever comes on according to him. When he blew it up I asked him if he saw the light come on and he said no. I will test the fan and light today. if you fire it up with the radiator cap off can you see a good circulation of the coolant like a car? Whats a good way to check the water pump to see if its working?

The way you are describing it here, I bet big money the fuel pump flow rate is no good or he was running shiyty fuel with low octane.

1) Do a fuel pump flow rate check.
Remove the line from the carb and stick it into a measuring container.
Using the starter turn it over for ten seconds (use a stop watch). NO IGNITION - you don't want the machine to start up on you.
You want 4.5 oz in 10 sec flow rate.
CAUTION FIRE !!!
2) Ask him what kind of fuel he was running. What octane ??
3) Best way to check the coolant flow is the way you described it above.
4) That 20/1 oil mix is WAY to rich. I bet it runs like crap. It should be 35/1 - 40/1. Maybe even leaner. Klotz recommended 32/1 to 50/1 for liquid cooled two stroke engines and 20/1 for air cooled.

This is from the Klotz pdf tech sheet:
2-Stroke: Air cooled engines mix at 20:1 to 50:1. Water cooled engines mix at 32:1 to 50:1
4-Stroke: Fill crankcase to manufacturer’s recommended level.
Kart: Air cooled engines mix at 16:1 to 20:1.
R/C Model: 2-stroke glow engines mix at 15-18%. 4-stroke glow
engines mix at 5-10%.

Pics, second set same thing?
Bored or not etc. Sized on the intake or exhast side.
Follow break in procedure.
Put piston in the correct orientation. Ring gaps to intake side. Sounds like a wiesco not done right to me. Big guess on my part.
Pics and more info will help.
What was your rings end gap when you checked it.
Bored to piston clearance was ? For what you have

I doubt it overheated. Look to seizures and clearance issues.

------------ FUEL ------------ !!!!!