Alan, for what it's worth, I put a used B35 with Motronic 1.3 (so the stock setup) in my coupe, from a 7-series. After checking compression and leakdown, I have been running it for ~15 years with no major problems to speak of. It did start to smoke a bit last year, I think the valve stem seals got a bit dried out and worn based on the smoking conditions. I used some of this stuff (
https://www.amazon.com/gp/product/B00CPL8WY8/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1) based on the feedback from a local indy, and it hasn't smoked since.
Other than that, I have had to do basically nothing to the engine. I did some work before putting it in the car, though:
1. New oil pump
2. Replaced all seals and gaskets, except for head gasket and lower timing cover.
3. Replaced timing chain tensioner
4. Replaced ignition wiring, rotor, cap, and coil
5. Replaced crank position sensor
6. Replaced idle air actuator
7. Replaced water pump
8. Replaced alternator with a later internally-regulated unit
9. New belts
10. Adjusted the valves
So I didn't touch the combustion-interfacing bits at all, since those are generally solid, and my compression/leakdown were good. Based on my experience, you don't need to rebuild the engine unless you really want to. Granted, that's a giant sample size of one, but that's all I've got to go on.