The chassis leg ONLY needs modifying if you are using some of the existing mounts.. this has been done in the past and this is the modification they have made..
Ive seen both of these 1.6 micras and ive converted a 2.0 micra aswell.. The best thing to do is fabricate custom mounts and driveshafts.. this saves on the integrity of the chassis
This is the best action to take in my opinion...
drop it in, in a decent position that leaves plenty of space all round, fabricate some temporary mounts up, then throw some shafts in that are welded to suit the position of your new engine and gearbox...
At this point check for cleareance on the shafts and boots etc.. and generally all round
- then if your happy you can then fully weld up your mounts and whatever bracing you want, wire in your electrics etc..
If at this point you decide you need to change things, then its not a huge job..