Removing the a/c compressor from a 1990 seville

The cover plate is not removable, You have to remove the whole fan end clutch asemmbly,

On a 1990, I would look at a salvage yard for
the bracket or whatever you need

You may be able to silver braze it (weld)

Replacement clutches are available and you can have the clutch replaced. However, there may be a problem with the compressor bearings and the entire unit/system needs to be checked as well.

Yes, you'll need the proper A/C clutch removal tool. The clutch doesn't seal the compressor.

The clutch and it's bearing(s) should be separately replaceable.

the ac compressor is seized up, replace it./

Replace the whole compressor. If you don't have the proper tools to replace the clutch with, you will never get it out. The compressors are bad for the front seals leaking on those, and would be best to replace the whole thing.

The compressor has become an integral part of the system to the point that there is no listing for optional no a/c. Looking at the pulley/belt arrangement, the only way you could do this would be to remove the compressor and replace it with another idler pulley. Eliminating the compressor, not using an extra idler and using a shorter belt would cause the water pump to run backwards (won't work that way). If the clutch on the compressor is OK why not disconnect compressor from the harness and just let the clutch freewheel without engaging?

Yes, look for an aircon technician that can swap a/c compressor clutch without removing the compressor from the bracket. if your lucky enough there are technicians who can do it without even leaking or discharging the gas from your aircon system

you have to replace the hole compressor and you can get that off by removing two bolt on the bottom and it is easer to get to the bolts from under the car.