Easy fix, for a master-welder.
Tungsten Inert Gas welding.
Done it several times, on PK Rippers mast's.....
Made an cupper Seatpost, so the weld does'nt stick on it, on the inside, off the Seatpost-Tube.
I'm in The Netherlands tho, any Alu-Welder can prob. do this job for you.....