Can’t help with the towing, but in an attempt to stop you getting fleeced…
What gen mx5?
Not sure about NC/ND, but on the A and B, if it’s the’fan belt'(alternator belt) as opposed to the cam belt, then I’m trying to figure out how the HG could be damaged.
The water pump is driven off the cam belt IIRC. If that snaps, the engine will stop before it has a chance to overheat the engine. It’s also a non interference engine so valves, pistons and head should be fine.
If it was the alernator belt, then iirc that only drives alt, PS and AC, and failure of any of these shouldn’t cause overheating.
On an NB and NA changing the alt isn’t tricky, it’s just a bit of a faff access wise, as the lower PS bolt can be chuffing tight.