Take a good, close, look at the inner structure. That should tell you a lot.
The RH (flywheel) side of the frame had a spacer at the upper mount, similar to the lower one. Figuring out the proper thickness and ID is dead easy. Set the engine in place, using the lower mount for holding power and the upper mount to determine bushing thickness & its centerline (once drilled). It's basic lathe work, if you don't happen to have the broken-off piece.
Usually, I find the broken piece. From there it's a bit of welding & metalfinishing. As for how it should be welded, bear in mind that this is structural piece. And, especially since it's cracked & broken free, tack welding isn't really adequate. You want the entire perimeter of the piece properly welded, for strength. Fortunately, that interior bracing (cradle) is thick enough to make the structural welding fairly easy and it doesn't have to be pretty...just solid. Only the outside needs to look good.
FYI, it's highly likely that this frame has small, horizontal, stress cracks, along the vertical sides of the triangle, above the upper motor mounts. It's a K0 and was abused. Those are pretty easy to fix, too, as long as one has basic sheetmetal welding skill. Usually, the cracks are tiny and only visible after the metal has been bead blasted back to clean, "white" metal.
Hopefully, this doesn't come across as being all that complicated. With a modicum of welding talent, it's really not that bad...maybe 2-3 hours work, including metalfinshing, tops. The biggest difficulty will probably be the lack of working room, inside the frame. By the time the repairs are done, the frame should be at least as good as new, possibly stronger.