If your current car is a mk1 903, fitting a 999 Fire is a fairly big job. Your best bet is probably to keep an eye out for a rusty scrapper 999 Mk2 then you can scavenge all the bits and pieces you need and have an installation to measure to see where to install stuff. It's a newer, unrelated engine so it won't marry up with the gearbox so ideally you'll need that, engine mounts, possibly driveshafts?? the inner CV joints are different anyway if you swap the 'box. Exhaust won't marry up either, manifold it doesn't look like you've got there so you'll need that, and the standard mk2 exhaust is designed for the coil sprung rear so you'll have to graft the Fire front end onto your existing rear - maybe a Fire engined 4x4 exhaust could be made to fit as it's more closely related underneath. As above for ease of running you will need a carb - think that's the Bosch injection unit you've got there?
As I said, easier to lay your hands on a complete car to transplant - must be a few rusty ones out there.