Frame adjustment for engine I want to turn my bike into a motorized one, but i can't get engine clearance because the 2 tubes in the picture are so wide on my bike. I was wondering if I cut those tubes and replace them with thinner tubes will the bike still be safe to ride?
No.

Those tubes are what stop you hitting the ground - hard. If your modification fails you could have a very bad day, or a very bad (even brief) rest of your life. A competent frame builder could modify a frame if it's steel, but would probably find it easier to purpose-build a frame.