This was a problem particularly on the K0 diecast and K1 bikes. Honda's techs sometimes forgot to drill the 3mm drain holes near the forward engine mounts on those frames, and they rusted from the inside out. Some even broke. Sometimes, the holes got painted over. Sometimes, the holes rusted shut, then the frame rusted out.
Get a welder to lay a thin piece of steel over that area, weld it full length on both sides. Do NOT weld it across the tube, only alnog the length, as it will further weaken the tubing if it is welded across-wise (like on car frames). Drill a small hole (1/8" is fine) in the center of the new patch, thru the bottom part of the tubing. Then, spray a generous amount of oil (preferably not WD-40, because it retains water, but 10w oil is OK) into the holes, turn the frame upright, and let it dribble out for a couple of days before painting.
On the other side, check to see if there is a drain hole by the other forward engine bolster. If not, make one, about 1" behind that mount.
We fixed lot of these with a "quiet" announcement by Honda's reps in the day...
Another place to watch: on the #2 exhaust pipe, there should be 2 drain holes where the pipe turns up into the muffler. Chain lube often closes these, then the pipe rusts out. Check those, too.