Another option is modify a 240 or 700 alternator to fit the stock generator bracket. Takes ~1 hour and common home shop tools, total cost including good used alt' was under $25.

http://www.p210.homestead.com/210altconversion.html

Is the page I put together when I did the 55A alternator from a early 240 into my 210, I have since put one of the smaller 100A alternator from a '92 740 into my 1800.

Changes from the page include using a cut to fit spacer instead of a jam nut to hold the back of the alternator tight up against the front of the generator bracket, and using a heavy gauge wire directly from the alternator output to the starter. I used the wire from alt' to starter cut from a 240 harness, just the right length and heavy enough.

The 100A is overkill and there is a short shreek from the belt just after starting but some 700s have a similar size 80A that would serve your purpose. The nice thing about these is rebuildability and the fact that they are very common/cheap in the u-pull yards. The case modifications do trash them as cores but all they ever seem to need is a new brush pack every 150K miles or so, never had to use one for a core.
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Dave Shannon
Durango, CO
'63 P-210
'67 1800s
'88-240
'06 F250 Diesel 4X4
my pages