Some options (listed from easiest to most difficult)
1. Transom mount at the front
2. Place the trolling motor at the back of the boat.
3. Get some marine grade plywood and fabricate a mounting platform for the trolling motor. Shape it so that it creates a horizontal surface when bolted/epoxied/riveted/screwed to the aluminum boat. Seal the plywood with several layers of epoxy + catalyst resin. Bolt/epoxy/rivet/screw it to the frame. Then bolt the trolling motor to the flat platform. Instead of plywood, you could use HDPE (cutting board material)
4. With aluminum angle and marine grade plywood, make a front deck. Bolt the trolling motor to the front deck.