You may find that your MC18R5 cases only support neutral whereas the MC21 has a 6 position GPS (gear position sensor) cable coming from the gearbox.

I'm not 100% sure of this but your PGM3 may need that feed to allocate the ignition curve to each gear.

If this is the case I'm not sure if theres a workaround. Others may have tried this and have a solution. Perhaps you could somehow choose a safe map for all gears and lock that in with a bit of solder somewhere?