Old Britts had a couple listed for 1500 USD when they started clearing inventory. I believe they are both gone. But even at 600 GBP for shipping and using 1.25 for conversion it is under 1800 USD. Which sounds like a lot until you consider that they are ready to start bolting things on.
If the old frame is repaired it still needs to be prepped and painted or powder coated. This is no small task and it isn't free. I'm not sure which direction I would go but of course I would also consider that Andover-Norton sells all the repair pieces individually. So if the idea of straightening the frame is not appealing, it could also be cut apart with new bits welded in. I've not priced out the entire list but one could buy every piece required and hire a good frame shop to weld it up. I'm sure there are people here capable of welding it themselves. I don't credit myself with that kind of skill. I do have friends who weld aircraft frames. So it's something to consider I guess.
https://andover-norton.co.uk/en/shop-drawing/505/1971-on-commando-frame-repair-items
Additionally, I would wonder if A-N stocks these or whether they are welded up to order. If the latter, it might be possible to have custom modifications made to the order, such as a cross tube at the front of the frame for additional stiffness.