There are two kinds of front drum brakes: 200 and 228 mm diameter. The little drums might be a little bit weak but the big ones are at least as effective as disc breakes.
The axle stub - or as Malcolm says, the upright - from r5 doesn't fit. It's very similar an it can be mounted but the steering knuckle is different.
The conversion of the front end is quite easy. You have to change the complete axle stub including disc, calliper and brake hose. The master cylinder can be reused except very old models from the 60's.
The rear end is much more work, because you have to cange everything from the hand brake lever to the brake anchor plate. If you want to convert a older model where the seats dont leave space for a GTL hand brake lever, you can use the hand break lever and mechanism of a R6.