Forget rebuilding unless you can do it yourself- go with a used one.
The same MSG-5 tranny was used in all 2wd Isuzu vehicles with the 4ZD1 2.3L engine from 1986 through 1995. This was the tranny in all the base model pickups, the most popular model by far. Try salvage yards, craigslist & the like for a used one, I have found them for as little as $150, a tranny shop won't even open one up for that and new parts are extremely expensive and hard to find. Don't overlook one of the "2wd pickup, 5sp, needs engine-$400" type adds; rob the tranny, scrap the rest you've still got a reasonably cheap fix.
The MUA tranny will bolt to the 2.3L, use the 2.6 engine plate, modify the driveshaft connection, swap in all the hydraulic clutch components, or JLEMOND has a method to modify the MUA to cable clutch, don't know if it has been posted on the Planet but it has at isuzupup.com.