i have worked for the worlds largest a/c compressor manufacturer for the last 11 years, so am quite qualified to talk about this

the r12 is no longer legally available in europe.

your system can be converted to r134a, but it may not be reliable. depending on the materials used in the hoses and shaft seal of the compressor, the leakage rate may be significant.

i have just bought an old scooby that had an r12 system and i have converted it. to do this, i removed all components and flushed them through with brake cleaner to remove the old mineral oil. all o-rings were changed, as the old nbr material can split. new pag oil was added to the compressor. i haven't changed over the service ports yet, but doesn't matter as i have access to all types of fitting.then it needs a very long vaccuum, before filling with r134a

an r12 system filled with r143a will not be quite as efficient as when it had r12

if you remove the compressor, condenser (at the front) and the hoses and flush them through with brake cleaner, and flush the evaporator (behind the bulkhead), you can then ask any service place to refill you with oil and r134a. how long it will last i can't say without changing o-rings