If it’s T6 heat treated to regain full hardness it would need to go through the heat treatment process again, although just by age hardening, ie welding it and letting it sit for a few months it should naturally achieve around T4 hardness.
Edit: I’m not an expert, I just did a fair bit of reading up before having a 6061 T6 frame repaired, that’s still going good.
I’ve got a spare 07 UZZI VPX front end that should match up with the rest of your frame if all else fails.