If you've got the complete SE motor, then fitting the dry clutch to the R is a breeze.

You can get an idea from the engine rebuild documented on this site.

The only thing you'll have to remove from the left side of the engine will be the gear shift linkage and the front sprocket.

Need to drain both oil and water too.

Keep an eye on things inside the engine. there's a washer on the crankcase side of the water pump gear for example that likes to fall off and run into hiding under the bench.

If you've got a parts book (or just look at the parts section on this site) you'll get a good idea of all the bits and where they go.

Other things to pay close attention to is the countershaft seal when you're refitting the gearbox and sliding the countershaft through the seal (located behind the sprocket) because the splines on the gear shaft can catch the spring, or the lips of the seal and damage it. Just take care, keep and eye to it and don't rush it. grease all the seals to help it slide together.

Another thing is the gear selector return spring. This is on the inside of the transmission cassette plate and needs to be hooked up into position before sliding the tranny plate home or it'll get caught between the tranny plate and the cases. You can then cause damage if you don't notice and just bolt the plate up.

Basically, everything you take off the SE motor to get the gearbox out will then go onto the R motor.

Hang on, didn't see in your post if both engines are 21's or 28's. If they're both the same, either 21 or both 28 then no problem, but if one's a 21 and the other's a 28 then you'll have to remove the primary drive gear as well as this is different between 21 and 28.

There's nothing too tricky involved and the swap can be done in an afternoon, even for the first time.

You'll probably need a new clutch cover gasket though as this'll more than likely tear when you pull the cover off, especially if it hasn't been apart in a while. Grease up the new gasket and it'll be easy to remove next time.