Oy ve, have the same issue here, been offered a unique, one-off tandem for restoration, will take up a ton of real estate in the shop and actually getting it here will require a truck. But it's cool.....
I have a Cirrus with the same problem, though not as bad as yours. I had a welder run a bead on both sides of the crack, but of course with heat treated alu this is always a 'ghetto' fix.
Finally got around to putting this thing together, mostly DX parts with some Ritchey Logic thrown in. Has wear and tear but I thought I'd leave it in 'polished relic' condition.