I have been asked several times about chain driving axles. Here's my take on the problem.

The only chain driven Trikes I have seen are of three types:-
1. Rigid frame - the axle stays rigid in the frame.
2. Swinging arm suspension - the axle is mounted on a very strong swinging arm so that it can ONLY move vertically, resisting any body roll so that no twist is imparted to the chain.
3. Independent suspension - the differential is mounted on the chassis and the wheels allowed to move in any direction they like. The diff maintains it's relationship to the chain.
Any twist on a chain will wreck it instantly. Body roll is not desirable on a Trike anyway, it feels awful.
The axle I used was from a Reliant Regal, you can tell this axle as it has nuts on both sides of the diff casing bolting the whole thing together, the Robin/Rialto axle only has nuts on one side. I've never been inside a Robin axle so I can't help much there. I had to shorten one side of the Regal axle for my mkII Trike. These instructions are based on how I went about it.
Decide which end you are going to shorten it to align the chain and by how much. Remove the brakedrum, backplate and halfshaft, Scribe a line along the axle and acrosss the end casting so it will all line up later. By removing the 2 holding bolts and heating the casting (I only used propane so it wasn't that hot) I was able to pull the casting from the end of the axle. Saw off the required length, machine (or use a sanding disc like I did) the axle tube back to the correct diameter so it's suitable for shrink fitting the casting, then the casting can be re-heated and re-fitted to the shortened axle tube. (you did line up that scribed line didn't you?) Drill through the bolt holes and re-tap (5/16 UNF). that's the outer tube done. If the end casting comes loose you can tack weld it to stop it fretting (well it's held on mine).
The halfshaft takes a bit more work. I'll work on the assumption it'll be shortened by 50mm. Have a look at the picture below.

Scribe a line along the length of the halfshaft. Mark the centre of the bit you want to remove. Using this mark as the centre, put centre pops 25mm either side, put another pair of centre pops a known distance from the centre line ie 100mm. The second set of pops will give you something to measure to so you know you've cut out the right amount (or you'll know how much you've cocked-up by, always useful). Drill 8mm holes down through the middle centre pops and saw down through the middle of these holes. file out the 2 half holes to take a piece of 8mm square key steel. File the ends of the halfshaf flat checking that the distance between the remaining centre pops is correct (in this case 150mm). Turn up a sleeve to fit over the halfshaft bored out a couple of thou smaller than the shaft (1 thou per inch diameter is a good shrink fit) . I found the halfshaft I had was not round so I averaged the diameter and used that as the bore diameter for my sleeve. Re-assembly is best done with the halfshaft vertical in a vice. Weld around the end of the sleeve while everything is still hot. I put the shortened halfshaft in vee blocks and although I didn't clock it up the finished item seems to run true
Using this method the drive is not all transmitted through the welded sleeve.

On a Regal diff the crownwheel is rivetted on, grind the back off the rivets and punch them out. I had a new sprocket made by Talon Engineering in Yeovil, Somerset with a hole in the middle to fit the diff and 8 holes to bolt it on (they also made a new front sprocket to keep the gearing right).

I marked through the boltholes and drilled and tapped the diff M8 to fit the sprocket where the crownwheel came off. NOTE use loctite or similar on the sprocket bolts or they WILL vibrate undone - I know this from experience, and it means stripping the whole axle down again to fix it. I centrepunched the threads to stop them from undoing. The largest sprocket I could get in was a 33 teeth assuming a standard 520 type chain. I replaced all the bearings with sealed bearings, you will also need to come up with a way of holding the oil in. I made a "top hat" which just fitted over the planet gears, The turned face and diameter was to provide a face for a gasket.

Some people fit grease nipples, but have to grease the diff every time they go out. Remember if you turn the axle over you will have to swap all the brake mechanisms side for side or the brakes will be useless.

NOTE: Since writing this originally another (simpler) way has been developed The axle tube is cut at the position of a mounting point, another tube is fitted into the axle tube and the whole assembly welded back together. The joint is then strengthened by the fact that the mount is welded across the joint - see below.