Im not sure if heat can affect those modules or not. I have had a mower here with one mounted under the flywheel and it gave no trouble.
The electronic ignition did not come along till the series 80 motor with the external coil. The module was mounted externally under the flange that the cowl bolts too, above the carb.
The early ones were 4 times the size of the module you buy now so i doubt there would be room internally as a retro fit.
About your only option is to try another module.
You culd try swapping coils first. But the module is a lot easier.
