Alternator replacement ??

Unless you can track down a truck one with a higher output and the vacuum pump on the back you are stuffed as far as keeping it "standard".
Don't bother getting the alternator rewound to give a higher output as while you get a higher output it is only at the upper end of the rev range and when we tried it back in the 80's actually decreased the output at the normal diesel operating revs.
The other way to go is to get an electric vacuum pump and tank and use that for braking vacuum and fit a generic higher output alternator.
7XX series turbo Volvo's have an electric vac pump which will operate for years and a vac tank behind the front bumper if you can find one at a wreckers. They have an inbuilt "pressure" switch and you would probably need to use a small vac tank.
Alternatively you can modify any of the Thomas brand compressors (blue tongue, big red, the black and silver ones sold under different names) as they will suck enough if proper fittings are used on the intake side.

I would say you would be disappointed with a higher capacity alternator because your batteries would be taking as much current now with the current system as they would with a 120A model. If you had say 3-4 batteries though that were down on charge then the 120A alternator would indeed provide enough current to charge them all at once but a single battery, even when 50% discharged will only take 20-30 Amps max.

Just make sure you headlights are off as this alone will limit the available charge current by 25-50%.

Regards
David

There is a replacement with a pump included available, i fitted one to my mates 60 a few years ago, i cant remember what brand it was or how many amp output it was. 100a or thereabouts it was internally regulated. worked well charging 2 batterys,if you like i will ring him to check on its specifics. Paul