I think the 900 series started in either 1978 or 1979. Seen quite a few of that vintage... they look quite similar to 99's but when you double-look you realise they've got the longer nose!
I guess you mean you want to fit the rear hand brake and 4x108 PCD hubs and the later calilpers and discs? All of those parts are available from the 1988-onwards 900.
It's quite a bit of work and ideally you would have the two cars sitting side-by-side so you could swap the parts over.
You need to change the following bits:
1. Handbrake mounting assembly between the front seats.
2. Fit handbrake cables going to the rear of the car.
3. Change complete rear axel (roughly 11 bolts)
4. Change inner-drivers on gearbox.
5. Change front hubs for later type.
On point 4... As the brakes were updated for 1988 so were the drive-cups on the sides of the gearbox. You will have to change the cups on the gearbox. This will mean you'll have to drain the gearbox oil and make sure you set the diff backlash properly as you install the later drivers. If you need the instructions anyone with a copy of the gearbox manual can help... just ask.