I used to do a lot of 12 to 134 conversions with my best friend when I was a tech in my younger years...

Basically, to do it right, you need a new:
At the least...
Compressor
Dryer
and to do it really right add the expansion valve and condenser.
If if money is no object the evaporator too...but most can be cleaned with pressurized mineral spirits...no one likes to crack open the evaporator case LOL
The condenser normally can be cleaned with mineral spirits, but if you are dealing with a really bad compressor too, the seals are probably lodged in there so might as well get a new one...You can also soak the expansion valve in a cup of mineral spirits too.
You have to get all the R12 oil out. It is incompatible with 134 oil. If you don't, the oils together foam and reduce the evaporation.
Lines need to be flushed, but normally get clean.
This will get you close to R12 performance.