1997 BMW Z3 cat converter

Honestly, a clogged catylatic converter can cause other problems that will cost you. The O2 sensor you may get away with, but the fuel economy may drop as well as the engine's performance. If you disconnect the catylatic converter and put a straight pipe in its place, that would eliminate blockage, but its against the law.

My recommendation is to try to get the money to fix it correctly.

The amount of $ in your budget should be enough to cover replacement of both, there are several good aftermarket suppliers of direct replacement parts that are reasonable, and I too believe the repairs should be done correctly, no "test pipes".