You should do a compression test before pulling out much of your hair.
Compression test kits are cheap on amazon. Find the specs for your engine and see what you're getting. If the rings and or valves are shot, you're wasting time you could be putting into a tear down.
If compression is good and you know you're getting fuel (wet plug) it may not be firing under compression. I've seen brand new plugs do this. It could also be an ignition issue. Seeing spark doesn't always mean it's strong enough to run. Moving on from those, you head into timing issues.
Good luck, keep these machines alive!!!
I was born and raised on Venus & I may be here a while.....