To reflect what another fellow said- "he'd not do a 125 again".
No offense to those that love their 125 conversions, I'd not do a 125, full stop. Plenty of 250 2ts out there to do - even here in OZ , where they don't grow on trees like in the US.
If you're determined on doing this (fair enough, determination and desire achieve many things), give thought to replacing the entire down tube / Y. Sure it might give you some fun and games with radiator mounts, but that can be solved.
My thoughts are : either a 'downtube and Y, and potentially the forward part of the cradle' in Plates, or new full cradle going to the plate DT and Y.
Nah, bugger that, here's something Much Better : a full new cradle to the original down tube, that has the following = the least fabrication of all. You could have a U, in either plate, bent U, or cut section of square / rectangle, placed on the front/ sides of the original down tube, to 'straighten' the downtube, then attach the new full, double bend cradle to both the DT extension, and the original DT. The original lower radiator mount could either be left in place, as part of the extension interface, or added to whatever you did. The original / interface is probably the strongest option.
Here's some pictures of how TM do their cradle and down tube set up on their alloy frames, for an example of a"full cradle":
I'm curious why you didn't do the steel TM frame you cut up - I was more than willing to help with that, as I told you. You would have had something unique, and in my belief, better than what you will end up with, with a Honda conversion. I've ridden the TMs, and the various Hondas people base their conversions on, and to me, the steel TM frames, especially, I think, are well suited to having a 500 2t in them. Probably not the thing to say here, but that's my take on it.