Lodola flywheel and charging

I’ve been enjoying pottering around on the Lodola over the summer but now have a couple of questions which I hope someone can help with. The charging light used to go out as soon as the engine was running. However, the dynamo drive belt loosened when out last weekend and I took the opportunity to replace it,. Now the charging light only goes off at about 2000rpm. Belt tension is now fine. Any clues?
I also wanted to check if anyone had a tip on the best way to lock the engine to allow the flywheel nut to be tightened and what it should be torqued too.Â

I don’t have a clever way of locking the flywheel to tighten the nut but I recommend a “lock washer”
It’s not really a washer.
I used one of the threaded holes either side of the nut, which are for the flywheel extraction tool, to attach a thin piece of sheet metal with an allen bolt which I then bent over as a tab to stop the nut rotating.
I put a matching allen head bolt in the other hole to balance.
The flywheel had previously come off and caused an expensive chew up!
It has lasted several thousand miles with no issues.

Could the belt be a slightly different size / profile and slipping on the dynamo?