There has got to be help closer than I. Unfortunately I am in North Florida, Gainesville to be exact.
My recommendation is to get the wiring diagrams for all inter related systems. I will look on Monday myself. There are a number of issues that need to be evaluated. These all involve the inter relationships on many systems. Some answers I don't know. Such as what will happen if systems that won't be used are not hooked up.
The engine management needs vehicle speed. If I recall, it uses one of the wheel speed sensors from the ASR/ABS system. This isn't going to work with the ABS on the 88 model. This might be able to be worked around technically but maybe not liability wise. (This is a safety issue). It may also have criminal elements as altering a safety system has legal consequences. Lots of research. I would start with the wiring to the control unit. I would look at each wire and all its implications. The above issue is just one that comes to mind.
Modern control units have multiple limp home and substitutive programs built in. The lack of or proper order of inputs can cause control to change to fixed or limp home strategies. Luckily you picked a model before DAS.
Bosch Master, ASE Master, L1
33 years MB technician