trouble is r50 is very low powered so all brakes would be wrong, anything is possible with deep pockets ,, i've stuff all kinds in all kinds of cars in the past, from mk1 ford capri with a rover v8 and coswoth gearbox believe me thats one car no one should evr do such things with as they like swapping ends standard,
maybe engine and gearbox wiring and engine ecu from r56 then its working out the bridge between the cas unit and emls unit and bcm ,, that said could use the r56 ecu and get someone to switch off the anti theft program for it , better off finding a blown up r56 and stuff your engine and box in that,,
one thing i have learnt is what begins with a idea always turns in to a can of worms and loads of hidden problems to solve,, nothing a welder and some electrical changes should be able to make work... if were me i would find couple of 1300cc bike engines and gearboxes and turn the r50 into twin engine 12000 revs screamer
I think it would be an easy build for the most part. Using the trailing arms and rear suspension from the R56. But even if one would be able to slowly migrate the all the wiring from the R56 into the R50.
It sounds like the Crush sensors that are in the R56 that tell when to launch the Air bags which would be in different area's of the R56 with attached wiring would be the hardest part.
That would probably stump me. But if you did some searching and are willing to loose your ABS you probably could work around that crush sensor issue.
r50 subframe will get in the way of the n14 front pipe and the engine and box mount points differ, but if you can weld you could make it work perhaps, the r56 frame is different for rack, and the lower arms of the 56 differ to the 50/52/53, top mounts I think differ, but you could explore using 56 hub carriers and adapting the frame to fit and maybe get the 56 rack to work and lose the 50 PAS pump etc, shafts of either car fit either wheel bearing, which fit either hub style, so 56 shafts can work, slave cylinder and pipe is different so you need that, shift cables differ, floor shift might adapt, the AC pipe layout differs in that the HP pipe (I think) runs down the OS chassis leg of the 56 and NS leg of the 50, if you have it, and the front panel is all different so cooling needs improvisation probably which could be the easier bit, then as for running the motor perhaps a wire in standalone would be your best bet, the ECU master classic is cheap enough, having a PnP harness to suit the r50 etc but you'd want a wire in harness I guess for the array of different sensors on the n14 that the old car doesn't have. Never done it, no experience of wiring or tuning the mentioned ECU particularly but have one and am aware it can deal with a wide selection of engines including turbo etc so should be a viable solution, plus it's less than £700 plus loom etc.
or just sell your 56 motor as they get decent money if you can prove it's good, use the brakes off the car on the 50, all rear trailing arms, wheel bearings (better bolts), park cables, rear brakes, break the rest, and tune the 50, buy a 53 bottom end and tune it for NA or fit a turbo kit haha, or the charger setup which plenty have done, cheaper than the engine swap but a few people would like to see it done perhaps, well those less scared of the N14 at least ;O)
I think it is another case of making a tube front like a IMCA late model roundy round in the end. That is not that hard of a proposition in the end for what you are trying to do.
I would just get a AEM computer to run the whole thing at that point. It has just as many functions as any other stock computer. Whether it be for any type of fueling forced induction or Vanos, VVT type functions. And is plug and play and easy to follow. That's how I feel about serious problems like that. If you don't have the entire setup from the other car. Even if you did you would still have to set all the crush sensors on the frame rails converting them over to the swap car. And I have not tried that but it sounds like another abs wheel sensor cv shaft magnetic reluctor situation. Where you have to set the crush sensors in a certain way. ALL TO GET the car just to power up.
That would solve the engine mounting and powering up. At a price but if you know someone that welds it would not be that bad in the end.
With the R53 it seems like it is just a better deal to mount the engine in the rear of the car after doing a Porsche rear trailing arm setup. Which are a beefy'er version of what a F56 Rear trailing arm suspension is. After that just mount the radiator in the front and don't do anything to that. And mount whatever engine you want in the back. put a Holly sniper kit on a old rover V-8.