Factors to be considered when re-engining aircraft
This depends greatly on the engine. However, given the 737 MAX situation, I'll assume you mean re-engining with larger, more efficient engines. This still leaves a lot of room for interpretation- what aircraft are we working with? Something like the 757, with tons of ground clearance? Or something like the 737, with very little clearance? I'll answer both.
In the first scenario, re-engining a 757, there's not a WHOLE lot that needs to be changed. The engine mount/pylon, wing structure (to account for a heavier engine), and maybe the FADEC. That's really all I can think of. Maybe the fuel system.
The second scenario is where things get interesting. If you want to avoid a 737 MAX style situation, you'll need to extend the landing gear legs and find some way to stow them. If you can't find anywhere on the airframe that's beefy enough to accept that, then you'll have to find an ideal spot and reinforce it. Then change everything that I mentioned in the first scenario.
I'm no aerospace engineer, and I'm sure I'm missing countless things here, but these are all the major things that I can think of.