The approach
@robmobile73 suggest should work out in general. The bolting makes sure the motor mount rubberpieces and the bracket on the engine and the new bracket (3x3/8" piece) for the frame are mating flat. The tricky part will be the angle between frame and the new bracket.
Not very likely that will be 90° straight. But you must not bend the rubber and you can not bridge much of a gap in your weld.
In order not to bend the rubber, make sure the engine is still hanging and new mount to frame are barely touching. Do not have the engine squeeze down and deform the rubber. Will be a lot of wiggling on the crane.
Depending on how it works out, you might need to flatten or sharpen the angle of the 3x3/8. A solid anvil and a decent hammer does the job. (Maybe some heat, too, but don't forget to quench in water then to maintain hardness).
But welding things like this requires some practice and some training. Burning a hole into the frame at the place it requires the most strength happens quicly when welding 'hot'. Welding 'alot' but only the outer edges may also create stress in the metal, making it prone to cracking.
There are techniques to prevent this.
Buy any chance: Try to get a skilled welder for this job.
You are doing great! Good luck Ralf