hi mate

well, sounds like a complicated one.

Firstly if your losing the power when you try and drive etc it sounds like something is loose, and movement is making circuits go open.

for a start, i'd replace all those charging leads etc that burnt, they may well be insulated again, but if that main charging lead has been shorting on the exhaust, its going to of been arcing.

I would also check all the main power supplies to the fuse box (Including that SHITTY fuse link wire, its a thin black wire intended to work as a fuse but on these old cars now theyre usually all corroded, or hanging on for dear life where some rough C*NT has been ragging it about)

things like that earths from the engine, and the headlight earths shouldnt really have any influence on the charging system, or running of the vehicle. Not to say dont repair them but i'd look elsewhere before u get too involved.

you could also check the main earth from the engine to the battery (the black one the same thickness as the positive starter wire) is bolted up nice and tight, usually on one of the starter bolts. check theyve both got good continuity aswell.

Try this and let me know how u get on, could be one of a million things being honest, but these are the areas i would of thought ur problem lies. Ive wired a couple of my own cars, one of which being an XR3i MK3 cabriolet which now has a series 2 lump, but has its original loom with the S2 wiring patched in and made to look like a whole loom, so i like to think i know a bit about the wiring on these cars

give me a shout if you need any more help mate