To put it simply, most off the shelf mounts are to weak to hold a gas engine. Electric uses thrust not prop torque to move the craft. This is why a flimsy rod holder mount that works on electric setups, will get destroyed with gas power.
Personally I have built very simple mounts using crates, bought rod holder mounts numerous times, and finally did enough research and spending of money to realize that companies do exsist that have a bigger R&D budget and their mounts actually do what they say and get results. Railblaza is the first easy on and off mount purchased that is store bought and works. Superior materials and proper coatings on the metal allows this mount, found below, to stand up over time and can mount up to a 2.5 hp gas engine safely. The mystery was finally solved.
Once mounted, offset weight or engine side flotation is needed to balance the 23lbs.
If store bought is not the answer, then keep on reading. Each mounting system needs to be thought out completely before you take on the task. From mounting positions, rudder usage, float type and height, to creating something that is effective and portable, all need to be thought out in advance. I have built over 6 different versions of mounts that are solid and perform, but every boat design is different and some have been an expensive epic fail.