Terry it normally wouldnt be much of a problem to get your vehicle insured,
(not for a 17 year old driver tho because the conversion is an performance enhancement)
A specialist broker like Premium Choice or Adrian Flux,
will ask for an engineers report, & a full detailed modifications declaration.
They will sometimes have this approved by an expert, in the case of a larger engine if its for a road vehicle, they possibly expect there to be an upgrade of the brake system.
There are other brokers/companies that are clued up & deal with classis,military,kits,custom,modified vehicles, & take it in their stride.
They may insure vehicles for shows without road use being covered.
Is your vehicle ready for going on the road & was it the one you were re-building with another chassis?
Is it registered, Engine number changed with the DVLA etc,(reciept for engine)
MOT'd & taxed or on a SORN.
So that when the Insurance run the Reg No. the vehicle comes up with them.
I really only see that someone else might need to insure it for you.
You may be able to drive it, or the insurance company might make an exclusion,
'no driver under 24' type thing.
The new rules of insurance for young drivers,
mean that the excess is ?500 for 17-20 year olds.
Best just speak with a few specialist brokers & see whats what.
george