Hello all, I am new in this forum, I live in the Middle East (Lebanon), we bought a 300CE 24V like 8 years ago and still with us from then. After three years, my dad decided to overhaul it entirely and we installed the 1992-93 hood, headlights, rear bumper and the license plate metal thingy.
We also upgraded the wheels and tyres from 15" to 16", and I found a set or (rare) red taillights and front diesel fender (air vent near the signal corner), so I thought it would look . And to wrap the thing up, we bought the Lorinser half wood half leather steering wheel with its airbag and automatic handle. The leather was also redone, half cream half black with the S500 design.

The car came from factory with Airbag, Auto, A/C, Headlight adjusters (from inside), ABS, Electric widows (all 4), Sunroof, Electric seats with memory, electrically adjustable steering wheel, etc...

Two years ago we dealt with two major issues: the car's computer and the engine really messing up. For the car's computer, my dad had the engine washed at the local gas station, it just didn't run again so we figured out that the computer was messed up and we had Bosch service center to install that of the 300E but the 5th gear is not shifting (we couldn't find another computer over here). So I need to ask what to do in this case? is there some "add-on" for it (that if someone had the similar problem) that will make the 5th work again?
And concerning the engine, well the old one cracked and the oil just got mixed with water, so we got some Hamburger sauce at the end . My dad replaced it with another one, but I am still afraid to loose this one also.

Sorry for the long post guys, Pls advise me what to do in both cases.