After a day or so of discussion on the pros and cons of this specimen, I was motivated to warm up the EagleCAD and came up with this:
(Rev.0 schematic deleted)
Update 23 March - Here's Rev 1, with several changes. I've added the option of a 20A main switch, and restored the self-test for the oil light (different modes for push start and electric start). There's a few other changes; everything new is in BLUE.
Thoughts or comments?