This was a struggle at first, nobody would touch me with the engine change, not even my current insurer who OK'd the change mid policy (Elephant). I was trying to get decent priced insurance for my Alfa 145 with a Coupe 16vt engine with a few mods:

Stripped interior, 2 seats + 4 point harnesses
Brembo calipers, bigger disks
Suspension
Roll cage
Air filter, exhaust, chip, BOV, alloys
Engine

27, 8 yrs NCB to use on that policy, 2nd car, 1000 mile limit all for £389 TPFT with Herts Insurance Co (www.hertsinsurance.com)

I've used them before on kit cars and they've been great, probably worth a call if you've got a few mods.