Mine have 486DX33 without jumper, bus is fixed to 33Mhz, if you put a regular 486DX2 it work perfect at 66Mhz, no way to change the bus to 25 (and not need to do). In the manual i have read support 8k l1 and 128k l2. I have only one socket for normal (non overdrive) CPU.

Is this your board?
https://stason.org/TULARC/pc/motherboards/I/I … l#ixzz2AtzV2UgX

You should have a plenty of upgrade options:

1. 486DX2-66 (normal or overdrive any brand).
2. Intel DX4-100 overdrive (either ODPR or ODP).

With properly set 5V -> 3.5V converter you should be able to use a range of normal CPU's:
1. Intel DX4-100, AMD DX4-100, AMD 5x86-133. All in WT mode.
2. If lucky even Cyrix 5x86 GP100

However, nothing is guaranteed until you actually try it...

i will check but i think that the l1 limitation is more than real

AMD Am486 DX4-120 has 8KB WT and WB variants, I don't think any faster CPU has less than 16KB cache.

Yes. Definitely try it.

OEM systems can be plagued by very weird compatibility issues and IBM are the worst. However, I'd be very surprised if L1 cache size was the problem. It's usually something else.

The Intel OVerdrive DX4 BOXDX4ODP100 CPU should work. I was able to get to work in my PS/1