Hello, Warsteiner the ideea is to keep the engine stock(i mean after i bought the car engine was opened and we changed just he main crankshaft bearings everything was ok so we did not modify anything i was planning to do a stroker but parts were not available ) Now the plan is to raise the power using "bolt  on " options as much as possible so plan is :
- ITB 
-some aftermarket ECU  i found this? 
https://lowcostracingsolutions.co.uk/pnp-standalone-ecu-s/56-119-bmw-e30-e36-e46-m50-m52-m54-m43-m42-plug-and-play-standalone-ecu.html#/39-accessories-just_ecu and Alpha N
My greatest fear is that i will not find some one that could program the ECU , i mean know "tuners" but this guys worked mainly on M20/M50 S52/S54 since the common practice was to throw the M42 at garbage and they are not very keen on working on this motors , usually they are saying get a 2.5 M50 and you will have the same power as an " tuned M42" 
So to recap : bolt on's , internals = stock i  camshaft will be stock if possible first step - ITB's then if all goes well  Alpha N and ECU edit : well looks like Dbilas have also Alpha N 
https://dbilas-shop.com/en/electronic-system/engine-management/alpha-n-control-unit/bmw/2649/electronic-alpha/n-control-units-bmw-1-8-16v-m42b18-1.9-16v-m44b19?c=13217