Depends what sort of bike, if this is the CB400 in your post can you try getting it on classic bike insurance as it's over 20 years old (some companies class 25 years as their benchmark for classic-dom).
Is it going to be a toy or your daily ride? If it's a toy and you're happy to restrict it to 2-3k miles a year that might help as well.
I used Carole Nash for my T140 Bonnie and they were fine with the mods, but Adrian Flux are known for taking on the more exotic machines.