New builds. Whats the regs these days

I'm planning to put my ironhead motor into a new gooseneck, imported frame. As the current frame and engine number are the same, will I just need to change it on the V5?, or am I hoping too much for a simple soloution?. Would I be better off with an engine from an unknown source? Years ago, this used to be a nightmare to do. I've known people build bikes for a long time, years sometimes, only to have it inspected and refused a logbook. I am going back a bit though (25+ years) What's it like nowadays to do a build?. Cheers, Cozz.

[Kiwi]

can you use your existing frame number on the new frame this would be a lot easier as you remove DVLA out of the information loop

Possibly, and there is no trace of the frame number on my existing frame as it was powder coated 20 years ago. Illegal though?

Slightly different circumstances, but if you have a old Landrover (Series type) and replace the chassis with a copy of the original, you can stamp your old number on and no need to tell the DVLA.
The way I see it ( I aint a VOSA man though), is that you are reusing your original lump so the frame must be almost the same as the original, now if you goosenecked your orignal frame and then hardtailed it, you wouldn't tell DVLA about it, would you?
I would suggest you either keep or detroy your original frame though.