If the plugs are new and you squirt starter fluid into carb and it still will not fire then you have a spark problem. Pull the plugs and recheck that you have a nice bright whitish blue spark. If it is an orange spark then you have a weak spark and it might just not be strong enough to set off the explosion. the coil, or the amount of voltage going to it, would be suspect in that case. Make sure that when cranking the coil is getting a full 12+ volts. Anything less than 8 volts is insufficient. Once the motor fires it can maintain on 8 volts.
As for the key not working it is a matter of hooking the right wires to the right terminls. For this you will need to determin which connection goes to where. You should have Off, on/run, start, common 12v+, and it should have negative continuity to the chassis.
Please see drawing for wiring information.
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Ok the only thing that might be screwing things up would be a capacitor discharge ignition firing at the wrong time. If the unit doesnt have a CDI ignition then you need to look at the fuel system better. If the unit bench fires ok then the common denominator between the two setups has to be the problem.
Try setting the motor up just like he did when he bench tested it, but in the frame. Then start to change the systems over to the tractor system one at a time until it fails to start. That will be where your problem lies.
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Ok, so you have it running then?
If he installed new wiring then it should be easy to determin the existing wire that is defective by doing a simple continuity (or lak thereof) and pinpoint the bad point in the wiring or the componant that has failed. If the failed componant is a safety feature then I strongly recommend that it be properly repaired and not just bypassed. There are reasons that these safety features are in polace. Ususly because someone at some point has been injured or killed.
Safety is something that should NEVER be bypassed.