Has anyone had broken motor mounts before.... The bottom set of mounts on my yz250 are broken off. Pics will come later.
So what....did the bolt break or did mount break at bolt hole? Or did the actual mount bracket break, if the first happened then I would suspect a loose bolt causing stress on motor mount, if the latter, then I would figure a combination of vibration and metal fatigue to be the culprit. Of course there's always the "shit happens" explanation.
do the mounting holes look elongated?
holes were all still perfectly round? lay them flat on a white piece of paper if you need to
how does the one holding the head look? and how does the hole in the head mount look? Usually when you get breaks it is due to uneven pressure or loose mounts letting the frame flex on hits and landings. those can, believe it or not wear out head and base gaskets from movement. One of the reasons some of us like the oringed heads better. your vintage of rig is one that I have yet to see break a motormount like you are describing. By all means when you get it welded make sure it is a two sided weld, and if at all possible complete the outside weld with the engine in place and the bolt torqued to spec. The frames are made of pretty mild steel contrary to popular belief and don't require any tricks to speak of, but you want them to go together with as little stress as possible to prevent that from happening again.
Those were pretty far past anything I have seen from that frame, too. Bikes are so much better and have been in the last 20 years that we see so many of them being used and used often for much longer now. We used to get a new bike or fresh frame and slide the axle as far to the front as we could and measure from the front axle to the back, or to the swingarm pivot or both. then from the swing arm pivot to the shock mount(s). 6 months and 15 to 20 races you could do it again and think you were measuring a different brand nearly. About 1990 for red and blue you quit seeing that so much.I think I would measure the motor and use all thread across the frame with nuts to lock it into place, then weld. Everyone has a way to do it, but yah, I have never seen any of mine in that bad of shape.
Good luck.