I can think of SUMO but he gave up. Building a custom frame (or modify beyond regulatory limits) is an expensive hobby in Holland, wonder what it's like in the UK.
'92 Enfield + Hatz 1B40: street legal, weld up stainless exhaust, check engine rpm and change final drive sprocket.
There are various web sites that go into the jigs needed, and how to do it. Mostly chopper sites, but the process is the same no mater what style. No links for now, I'm on my cell phone and need to get out welding in the garage.
In the USA, if you start with a frame with clear title, keep all receipts for everything you do to it, no problems. Can be more hastle if you only have a manufacture origin title on a frame, or make it completly yourself without pictures and receipts to the point you might not get a title and tags. So, yeah, paperwork will be more frustrating than fabricating it.
In the UK a custom frame generally means SVA. Someone put a link on here a while ago with the points system they use to work out if a bike is new or not - I think that basically defines if it needs SVA too. There may be ways around this although I don't know them, perhaps someone else on here is more familiar. Best bet is to have a chat to someone in the harley/customs building trade as they seem to know the in's and outs pretty well.
SVA in itself probably would not be that big a pain (I have a copy of the manual and it looks ok-ish) though I think you may end up with a Q plate, making insurance pretty um interesting...
n
Smart engined 800cc turbo diesel triumph tiger. 100mpg (imp)
Belfast to Kathmandu overland, 2010/2011 - http://www.suckindiesel.com
Bangkok to Sydney ???
There must be a way to keep the original numbers though. I am pretty sure I've seen customs running about with original plates but the likes of an inline 4 in a hardtail frame etc. I'll ask the builder guy I got some of my bits from for some enlightenment.
N
Smart engined 800cc turbo diesel triumph tiger. 100mpg (imp)
Belfast to Kathmandu overland, 2010/2011 - http://www.suckindiesel.com
Bangkok to Sydney ???
I think if you can unbolt the bits you dont want, and bolt the bits you do want on, in other words can it be just reassembled back to original then you get to keep the original plate. Look at the trikes plenty with original plates.
There's a nice old basket case BSA B44 on eBay at the moment, I think it was going for £166 so far.
Anyhow, hard tail, historic tax, would make a nice bobber.
There have been two Ariel plunger frames up for grabs on eBay over the past two months, if my garage didn't have so many old Enfield projects I would have bought them.
There are custom frame builders that sell on eBay should your needs get urgent. But there's nothing like walking away from the post office with a historic tax disc.