Using the motor as a stressed member requires that that the motor be rigidly attached to the frame in at least 3 places (head steady doesn't count) and that all mounting points (bolts) be torqued correctly and kept tight (check them often). Otherwise, your top tube will be doing all the work and will very likely fail. The "grass track" frame shown in your first photo appears to have a hinge in its lower connection which would eliminate its function as a stressed member.
If you're using a straight single top tube, be aware that compared to 1" x 1/8" wall, 1-1/4" x 1/8" wall will be twice as strong against buckling and 70% stronger against bending. Furthermore, 1-1/2" x 1/8" wall will be four times as strong against buckling and 2.7 times stronger against bending than the 1". Don't skimp as the weight penalty is minimal.
If you're using curved tubing for the top member, it becomes more difficult to analyze as all members will be subjected to bending. Also realize stresses will be approximately proportional to the square of the length of that member. Therefore, doubling the length of a member will increases bending or buckling stresses by a factor of four.
I am a mechanical engineer, but not a structural engineer so I won't try to give you any specific advice.
Tom
