Yes, you most definitely need a new compressor and a new one, but it doesn't stop there. After a catastrophic failure to the point of compressor lock up, you must break all connections and THOROUGHLY flush each line and component, then flush those components, then flush again. Flush, flush, flush........... It also wouldn't hurt to add a suction side filter.
The reason for all this is that upon internal compressor failure, bits and pieces are scattered throughout the system and if they are not removed they will destroy the new compressor in very short order.
In addition, you will need to replace the receiver/dryer and of course all o-rings.
On a system that has been converted to 134 you never know how it's been converted. An improperly converted system is destined for compressor failure such as this. In come cases even a properly converted system will result in your problem. The R4 compressor that is on your car is not among the most robust.
Some cars convert pretty well and some don't. Yours was not a good candidate. R12 prices are coming down. If the car were mine, since you have to thoroughly flush, replace the drier and compressor, I would put in mineral oil and R12 and reverse convert.