A few random thoughts on this topic :
I don't have any experience with rewinding the alternation, or have I tried a mosfet reg/rect. The mosfet is probably a better design, as the old style (like original) effectively use resistance to control the output voltage, so they generate a lot of heat, and subsequently fail, so they're almost a consumable part.
I have replaced my reg/rect twice, and the first time I replaced the two wires from alternator to reg/rect.
As I understand it, the single phase alternator on 851/888 is 350W output, so rough calculation of current required at 12V is 30Amp. So, I would recommend using 10 gauge wire (rated at 29Amp I think) between alternator and reg/rect for the old type, not sure about the mosfet type though.
The wire on the battery side does not need to be anywhere near as heavy gauge. 5 Amp wire should be more than enough.
From memory, the alternator should output about 30 - 80V depending on revs. You can test that with an AC voltmeter, and perfectly fine to test in isolation without reg/rect connected. Never test reg/rect without battery connected, you will kill it immediately.
I like to use the EDL450 reg :
http://www.ebay.com.au/itm/251993490618 as it supports the low charge dash light, where as most other reg/rect devices do not.
You should see up a maximum of about 14.4V from a good reg/rect as your charging voltage.
I'd say, first step get the alternator replaced / rewound, test it to prove you're getting the right voltage out of it. Buy a new battery, and your choice of reg/rect and go for it.
I hope that helps.
Cheers,
John
