Any question answered...

Can't be that hard to get, lots of stuff is/was built from 531, not just motorcycle frames but car spaceframes and aircraft parts (particularly engine subframes). There is a dedicated contact at Reynolds for motorsport projects.

On the other hand, it's an expensive proprietary product, and it's advantage over much cheaper generic 4130 CrMo is higher strength. In many applications, strength isn't the the overriding need, and once you've used enough 4130 to get the stiffness you need it turns out to be strong enough too.

At the mo it is nothing more than design/material stage, been looking at a few custom bike makers frames and quite a few seem to use 531 while it has been superceeded for tensile strength at thinner tube walls. So would the later tubings be cheaper than 531?

Needs to be steel as it will be welded in air so titanium was out, maybe stainless steel...