Thanks for all the responses guys, I've been corresponding more with the company as you can see! They've proposed doing a CAD drawing of the original rad and working from there, they make radiators for lots of late model MX bikes so they seem to know what they're doing.
As you can see my favoured direction is a deeper-than-stock single rad to retain the autolube tank on the 125 (this would also allow retention of the tubular steel radiator guard which most MX bikes don't have) but if they can do this there's no reason why they can't make some DT200R/Lanza sets as well for those who want to go the twin rad route.
This was what was recommended to me and did nothing but leak. I've used it on several components and never had success. As I say, the bond stuff seems to be the goto kiddy everyone is using these days. I recently redone my DT engine with the Hondabond and it hasn't had a drop since.