It depends on the insurance company. They seem to have different views on how mods affect liability/risk.
Generally, performance mods tend to be the ones they look for. Another aspect is that claims for damage might be restricted to standard parts costs. Expensive after market parts may not be covered, just the equivalent standard part value.
It is always advisable to ask the insurer. I have never had problems as never gone in for expensive mods.
For a Morgan I think the specialist insurers will be more amenable and understand the mods situation than a general insurer. The specialist insurers would be aware of known aftermarket suppliers as being good as opposed to non specialists suppliers.
