Now, I had my base car stolen, and chopped up for parts, so saying that it isn't a target, well, that's not exactly true.
But one of the largest concerns for a conversion is the idiots who think they are prime targets for vandalism. Don't know exactly what possesses them, but I have heard many reports about conversions getting hit harder than other cars, so a security system would probably still be a wise choice.
Now, as for other questions about any united states conversions. The guys at Knight Registries would be best suited for answering those questions, but if you aren't going for a 'perfect' replica, they will give you a little bit of grief. You also might want to try Knight Replicas, they seem to be a little more open to less 'perfect' conversions. You can do whatever you want, since it is your vehicle, however, but the more extraordinary you get outside the standard established conversions, the more you will find yourself on your own.
For UK conversions, you might want to go visit Knights of England at
http://www.knightsofengland.fsnet.co.uk/ they, too will have a lot more information there for you.
KI