I can now report that I have fitted an Electrix World ST5051L stator and a HPI CD5004 CDI and coil to my 87 Husky 510 replacing the inverted SEM ignition. It was a pain to instal as the mounting plate the stator comes with does not fit the ignition cover. It is designed for the non inverted ignitions. So despite what the Electrix World website says it is not a direct swap. I asked several times before I purchased it for both Electrix World in the UK and Horse Power Ignition in Belgium to confirm it would fit. Electrix World never bothered to respond and HPI said it would work OK. Turns out it works OK but need modifying as below.
1) spot the problem with the mount
2) You need to unscrew the stator from the mount plate. This is not simple as the M4 screws are quite tight. I used pliers to turn the head of the screw. Also need to cut some insulating epoxy away from one of the screw heads. It was quite tight to pull off the mount plate as there was some adhesive on mine. Be careful to apply force by putting your fingers under the lower lighting coil and prying the stator off the plate. A tap with a plastic hammer to the mount plate may help. Also be aware the upper ignition coils may come away from the lower lighting coil - they are separate items that are clamped together by the mounting screws and some glue.
3) You need to remove your old SEM stator from its mount plate. Drill out the riveted ends
4) Use 40mm long M4 screws and nuts to bolt the stator onto the old SEM mounting plate. It bolts through the holes where the rivets were. It's a perfect fit. Note you need to file the screw heads so they do not stick out into the bore of the stator. See the second photo.
5) Bolt the stator to the ignition cover. I removed the white zip tie you see in the photo above that was used to tie the wires together at the base of the stator as it was putting force on the wires when the stator bolted to the cover. I also removed a small ridge on the SEM plate that I was concerned may damage one of the wires.
6) Fit the ignition cover to the bike. I found the ignition cover would not sit properly and ended up packing it out by using a 0.8mm thick gasket to get it to sit properly. The Electrix World stator seems slightly taller than the SEM and I found it was rubbing on the flywheel tapered hub. Packing out the cover or else skimming the mount plate by say 1mm will fix this problem.
7) Fit the CDI and coil. This requires making some simple brackets and a lot of cursing as it's a tight fit under the tank on the single shock frame. This took me ages!
8) Start the bike and grin like a fool because it's got a nice fat spark and will start hot and cold. I found it now will start on the choke whereas with the old SEM fitted it would never do that - It didn't have enough energy . The bike now starts well, idles reliably ( it would stall at the drop of a hat before) and seems to run better but haven't track tested it yet.....can't wait!
I recommend the mod but it would be easier if Electrix World supplied it with the correct mount plate for the inverted ignition. I was a bit reluctant to pull to coil off the plate in case it didn't work after I was finished!