RENIX used the Delco CS-130 (six-cylinder, some four-cylinders used the CS121,) and it's a dirt common unit. Upgrade parts should be easy, and any alternator/starter rewind house should be able to upwind yours without too much trouble.
The shop I use - San Jose Generator - has no trouble getting a stable 140A out of the CS-130 case, Rod builds them all the time.
You can go higher (and stay stable, and below the MG $400 price point) by going to a CS144 case - find these in some full-size RWD V8 passenger cars (particularly Cadillac) and later GM trucks
Anything in the CS series can be set up as a "one-wire" job as well - meaning the regulator senses system voltage internally and is self-exciting (you have to "blip" the throttle after start-up to "wake it up," but that's done and over with quickly.) I am unsure of the newer AD-series units can be set up as "one-wire" units - but that's largely because no-one has asked me to check into it yet.
I'm told that there are CS144 units that will drop right into the mounting points of the CS130 as used on the XJ, but I don't have a list of applications to hand (the core and case are essentially the same, you just have differences in mounting points.) However, if you are fabbing up bracketry, get something simple (like a 180* mount) that you can find easily and go with that.
The later ND, as mentioned, will not work without the addition of an external voltage regulator (the ND is regulated by a circuit in the PCM on 1991-up Jeep, while the 1987-1990 Delco CS-series and 1986-earlier SI-series are regulated internally. The Delco is also more rugged (usually) than the ND - and, in my mind, installing an ND is a step backwards.
If you're going to go with a higher-than-OEM output alternator, upgrading your wiring is a good idea. If you hit me backchannel, we can work something out for you and your changes that will fit like it belongs there - and have it fused for safety (many people omit the fuses, most outfits don't provide replacements for the fuses - and therefore no alternator output leads - and every vehicle I've seen made 1950 or later has had a fuse or fusible link in the alternator/generator output lead, so I consider it necessary.)