Don't know if i's use the term "vintage' with the brand Spot. I agree with dsaul that the best repair is likely not worth it, replacing the TT and ST. BTW it looks like the crack is traveling along the weld of the ST and stay. Welding over the crack is one way to repair the frame. Know that some ST distortion is likely to happen and after welding seat post fit might need "massaging". Another method would be to braze on a patch over the crack. The patch would be a piece of steel sheet shaped best possible to fit as tightly as possible. Either way I'd be tempted to drill a small hole at (or just beyond) the ends of the crack in an attempt to limit the crack's continued growth. Andy.