David, the best suggestion I can offer you is this.
Remove the bottom tube also and weld in a rectangular steel tubing I recommend 11 gauge steel 1-inch by 4-inch. then you can add a flat steel plate ( aka ) also known as a
skid plate. see pic:
Skid Plate:
If you are using a Honda Mini Trail 50 or the cloned version of this engine, you'll know that the four ( 4 ) bolt holes on the bottom of this type engine can be bolted to your skid plate, and the the skid plate can be bolted or welded to the square / rectangular bottom tube. see pic:
Rectangular Steel Tubing for new bottom tube:
When you cut the new rectangular steel bottom tube, make sure you leave a notch of steel at both ends for added strength under bottom bracket and at rear triangle to strengthen the rear triangle.
Don't take a chance the rear triangle welds could snap under stress with this type of engine so definitely add a little tab under the rear triangle.
When you've completed your welding of the new rectangular bottom tube, your frame will look similar this square or rectangular bottom tube frame below. Hope this helps!