Welcome here!
My advice: throw out the old 64K expansion and order
new 512K or 1M expansion from Saint!Anyway if you want to repair the original:
The original EXOS RAM test will test the first byte of the RAM segment, if this byte failed then the RAM not detected. If the first byte good the start testing, and if failed marked as bad. And you will see xxxxx bytes not working.
If one or more RAM IC totaly dead then the RAM expansion will be invisible for the original EXOS.
I wrote new RAM test routine for updated EXOS (2.2+, 2.4 beta 5 the most recent), it is will detect the faulty expansion if least one bit (=least one RAM IC) working.
For the first step need to run my test on your cumputer. Upgrade the onboard ROM (most recommended, because other EXOS bugs fixed), or also availeble cartridge version of the RAM test, or there is a loadable version (it is start with the Advanced RAM test mode).
If the missing 64K detected as faulty memory, then the problem will be one or more faulty RAM ICs. If this test also not detect it then the problem with the address decoding circuit.
If the problem with the RAM ICs then try to write/read bytes of the faulty segments, and identify which bits not working. It is possible from BASIC with SPOKE/SPEEK instructions, but more comfortable from a Monitor program, for example from ASMON.
I suggest check the wires and solders of the expansion connection for a contact problems.
74xxx158 and 74xxx157 are compatible in this situation. The 158 have a inverted outputs, so when accessing 0000h of the 64K expansion, the FFFFh accessed with 158 ICs, and 0000h with 157 ICs. But at this situation don't care. Both type used in Enterprise machines, in a different production batch.