Disconnect kill lead to ign sw,crank eng & check for spark,if you have spark now then problem in wiring to ign sw from coil,ign sw bad or problem in safety sw's or wiring harness on tractor.
If still no spark with kill lead disconnected,check that connection from megafire module to eng block/chassis is clean/tight.After that check flywheel/coil air gap,manual says .010"-.014" or check instructions for megafire unit,air gap may have to be changed,after that goto link below & read instructions for setting flywheel timing as same is adjustable on 243000 series eng.
If after disconnecting kill lead,checking megafire ground connection & flywheel timing & you still have no spark,remove coil & take to one/more local mower shops & have them check in a coil tester,if coil tests ok,then problem may be one of the following:
1-Megafire unit bad due to heat/vibration depending on where same mounted on eng/tractor.
2-Flywheel magnets weak,this is a somewhat common problem on these old briggs eng,might try changeing flywheel/coil air gap if magnets the problem,you might be able to get around weak magnets this way.