Hi mate,
with us it would rate on the group of the donor vehicle/engine so if for example you put a vr6 engine in a golf driver, if it was a standard engine it would be about the same as a vr6 anyway.
Most cosmetic mods make little or no difference.
If you modify the engine itself and increase the bhp (eg cams etc) then you will normally pay a loading for increased bhp, although with most our policies you get up to a ten bhp increase for free.
This is with us though, it will vary with other insurers
Hope that helps
Martin