Yeah, most rebuilt or new compressors these days indicate they are for 134 use, because they assume that's what you will convert to. But the compressors haven't been redesigned internally, they are the same as the OEM R12 ones, and will work better with R12. Your 126 came OEM with R12, and will function better with it.
The only problem is, THE REBUILT/NEW COMPRESSOR MAY HAVE BEEN CHARGED WITH SEVERAL OUNCES OF R134-COMPATIBLE PAG OIL. If this is the case, then putting it on a system full of R12 mineral oil will create a disaster - the two oils are not compatible at all.
Has your system been converted to 134 previously? I assume not.

So... if he just popped it on, and mixed the oils, then you will have contamminated the whole system with a mix of oils, and the only solution is to flush and clean out the whole system, decide which freon you want to use - 134 or 12, and then add the proper amount of compatible oil and a new drier. Depending on whether the old compressor failed catastrophically, you will need to flush the system regardless, to clean it out, and replace the expansion valve.

step 1 - if the old system failed hard - the compressor seized or broke, then the whole system will need to be flushed, new proper oil added, and the drier and expansion valve replaced (EX valve replacement job is the worst I have ever seen).
Step 2 - If the system is probably clean of old compressor parts inside, then find out if the compressor came charged with any oil - if it did you can bet it's 134 PAG oil.
Step 3 - if it came dry, you will be OK, but will need to add several ounces of R12 oil (do a search herein for qty - about 4 ozs I think for a compressor swap). Then add a new drier, and maybe an expansion valve (see above) and charge with R12 (I think it's 48 oz, but do a search).
Step 3 - if it came with oil, and says use 134 on the label, then it's definitely 134 PAG oil, and you just contamminated the system with incompatible oils. Flush the whole system out, add a new drier and EX valve, add the proper correct oil, and evacuate and charge.

If repaired properly, the 300SE R12 system cools VERY well. My '91 has the original compressor, and will put out 40-degree air when it's 100 degrees and 50% humidity here in Dallas.

DG