I have a 1988 garden tractor with a 18hp briggs motor, I only has spark on 1 side, I have installed new plugs but has not resolved the problem. I looked in the owners manual and it has a magnatron ignition with a trigger coil, how would I test the coils or troubleshoot this. Thanks in advance :)

if you only have spark on one side, it does sound like a new coil is in order. you can test spark with one of those testers that allows you to set the gap, not one of those light up ones. or ground to metal, turn over and check for spark may tell you. some shops may be able to test the coil.

Have you removed the plastic blower housing yet?
I've had two tractors in my shop this year with spark on one side only, caused by
mice building nests on top of the cylinder on one side, and chewing up wires.
Sounds strange, but it DOES happen!

With the age of this mower I would say the blower housing is made of sheetmetal rather than plastic. If this is the case, I would check the wires where they come out from under the housing. This is a point where the housing has a tendancy to cut/rub through the wiring over time which causes the plug wire to short against the housing.