I'd replace the stay. You could patch it up but thats bodging rather than fixing.
Its Max which is going to make that a bit more expensive or difficult I imagine
What frame is it out of interest? Lee Cooper?
Max tubing is easy enough to get still...max lugs are not!!
Post a picture of the damaged area mate
@PhilDAS started
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I'd replace the stay. You could patch it up but thats bodging rather than fixing.