Forget replacing the compressor (unless it's shot)...and, even then, an R4 is the only practical fit on our cars and it actually puts out greater volume than many other types.
First, I'd identify the refrigerant. If it's been converted correctly, they'll be tags and new fittings and you'll know. If there are not, you should have it identified (there are simple machines to do this that all A/C shops are supposed to have). Many conversions are half arsed and not labeled.
If it's R134a, I'd have it recovered, then I'd charge it with dye, a little ester oil (~2 oz.)and the correct ~weight and pressures of 134a, check aux. fan, clean condensor and evap(see archives) and "test" run it for a while as a leak check. Put a detector to it (if you, or a buddy, have one). If the whole system is a sieve and is just being recharged again and again, you want to know early on. Purists disagree, but this is my technique. If no leaks and it isn't a total dog at cooling, then recover, flush evap and condensor, new R/D, some people put a new TXV in (I disassemble, clean and adjust mine; don't you,.. unless....), correct oil charge, vacuum, and correct ~weight and pressures of R134a. The reason for the flush, even though it's cooling, is you don't know amount or types of oil in the system. Some conversions leave the mineral oil IN and it comes back to haunt you now, or in later repairs. If you knew for dead sure the history, do it more simply.
If it's a total dog, with your relative humidity, or shows leaks at condensor, compressor or evap(rare on 126) I'd think parallel flow condensor and/or reconvert to R12 and obviously some new parts to replace leakers.
If it's R12 now, I'd recover, check aux fan, clean condensor and evap, charge with dye, a little mineral oil and an enviromentally safe "test" refrigerant. Run it for a while,repair/replace for leaks, flush, new R/D, correct oil charge,vacuum, and correct weight of R12. Again, the whole flush dance is because of unknown history on oil.
If it's a hodge-podge refrigerant or refrigerants.....post again.
My test run technique is not how a pro shop does it, but, on a older, running system , I feel it's safe and catches a lot more leaks than just a leak detector or R22 and nitrogen with a sniffer. I've got two electronic leak detectors (one a Yokogawa) and I like them and use them, but I really want more on an old, old, unknown system like yours or mine, five years ago. On a known system, in good repair, I do it differently.
A side note on adding a little oil to the system sight unseen for the test.
Of the five A/C techs I am pretty well aquainted with, all say: when oil is an issue with a compressor failure, it is always because there is not enough; not because of too much. You can hydr(au)-lock a compressor with too much oil, but they've never done it (or so they say
). You can kill a LOT of compressors slowly with too little oil or very, very, very few, quickly, with too much. Too much oil, or mineral oil left in a R134a conversion, typically causes loss of cooling.I wouldn't have the whole flush thing as part of this if I didn't take the amount of oil seriously, but I'm practical and not a worrier, so.......FWIW.
In my car, (this is not a tall tale, honest
) I have frozen the evaporator at 100+ºF in Mohave, CA. I adjusted my ETR switch a little too low, I guess. I do have a parallel flow condensor
and I was running some tests with a "test" refrigerant at the time. The leak I was chasing was through the potting into the electric wires on the pigtail from the aux fan switch on the receiver/dryer. Try to find that one!!
Anyway, this is just how I do it. I have a LOT of A/C tools, so it's a fun hobby. No one get all in a bother over "test" refrigerants or what not. I'm sure there are other, better ways, but.