Joe,
A tech familiar with the compressor can pull the heads, replace the valves and any other internal parts, and replace the seal in about 3 hours or less.
If you hold 300 psi of nitrogen overnight that system will not leak.
For R12 you are likely to have low side pressures of 38 and high side around 160 so if it holds nitrogen at 300 you are good to go.
Congrats on the Cruise Airs.
Jon