2004 would have been the last time I had anything to do with this model so .................."caveat emptor".
PLA MOTOR may be the warning on the IDS but you need to take into account that it implies the system is INOP but not necessarily exactly the motor.
Throwing another motor at it would have been a bit optimistic without confirming that it is actually the problem.
You need to understand the "logic" behind how the warning is triggered which may be derived and another simple fault in the system blows the logic.
i.e. 1/ Oil pressure "green range" versus temperature doesn't make sense if your oil thermo-valve is slightly out and you will get all sorts of warnings due to the "derived" info in the software from "mapped" data.
2/ I have seen an engine changed due to oil filter bypass indications that was only a duff thermal lock-out on the bypass indicator. Nothing actually in the filter! $35.00 fix versus $25,000.00 after the engine came back!
As a 20 year old aircraft I would have first swapped the motor from side th side (if possible of course0 then bus (ring) it out for continuity and be checking the earths or beep switch functionality.
The assumption is that the motor does not actually work and not just a spurious indication?
While you are at it I would take a good look where the DAU lives and it's general well being. Some where prone to moisture in that area doing all sorts of crazy stuff.
Stabilant 22 Enhancer is your friend on any connectors which you would be aware of as you have PWC engines where the use is pretty much mandatory. No idea how it works but it does!
As somebody in Italy told me - "Agusta is like a Ferrari - the fastest way to get to where you will break down next". They both need constant "fettling" to stay reliable.
The AMM is only the minimum required.