First thing you need to do is remove EVERYTHING associated with that old alarm you said is still hooked up. I had a similar problem where my truck wouldnt start. Finally realized I left a relay still hooked up from the ignition kill on the old alarm. Truck worked fine for months up until this time with the old alarm hooked up, just all of the sudden quit. And this happened in the middle of the woods during a trail ride. I almost had to call a wrecker before I figured out what the problem was. Had me stranded for 2 or 3 hours. So I would completely remove the old alarm and go from there.

Otherwise, check your spark plugs and plug wires for resistance. Check your cap and rotor. Check all wiring connectors to make sure they are tight.