I know the first pic is not your boat but one reason for jamming is NOT using the special cranked shackle that Plastimo supply to attach the halyard. It can also cause the top cap of the swivel to undo 'on its own'.
The nylon bush itself is not usually the problem, it is corrosion on the surrounding alloy. This 'squeezes' the bush and causes it to bind on the rotating shaft. Ideally you need to get the bush out and put a coating on the surrounding alloy to try to prevent corrosion. It is probably worth...
You can almost certainly mend it quite easily. Open it up and you will find the windings on the stator. Almost certainly one of the very fine wires has either broken or slipped out of the slot that hold it. Either put the wire back in or remake the connection with another part of the wire...
Had a similar problem. I removed the existing roller but kept the 'frame'. Got the local stainless fabricator to weld up a simple extension channel that could be bolted inside the existing frame. Works well, has been in use for the past 15 years.
On a single bolt heat exchanger...
I normally remove the matrix towards the rear of the engine. I remove the pipes from the end cap, remove the bolts from front and rear end caps, remove end caps. hook out 'o' rings, push out matrix.
When re-assembling the heat exchanger I put the matrix in...
The slot in the first photo is obviously there for a reason, presumably to allow any water that manages to get onto the top of exhaust plate to drain away, I don't think it's a problem. Having said that, I can't recall ever seeing water from that point on my own engine.
From memory, the area...
Fitted one several years ago when the topping lift broke and I was unable to, easily, replace it.
It's been great, does exactly what it should and doesn't look like wearing out any time soon.
I still haven't managed to replace the topping lift...