I was working on my '70 BSA this evening in the basement when I looked down and noticed a little bit of rust on the back side of the right down tube, just above the forward engine mount. There's a crack in the frame! Is this something that any competent welder can take care of? How screwed am I?
You need to make sure that the frame is indeed cracked. If it is you have a problem.The reason is that the engine mount lug is brazed to the frame and you can't weld anything that was previously brazed. So the fix would be to replace a section of frame tubing and then braze the engine mount lug back on.
One thing that I noticed is that the frame tube is waisted above the engine mount lug. Possibly crash bars or highway pegs were mounted to the frame tubes which could have stressed the frame causing the frame tube to crack.