Vertical climbing tank/rover
Not an engineer, but a biologist - so here's my proof of concept for what its worth.
Take a flat wood plate. Put a motor mount on it, tank, throttle servo, engine, protective cage. Weigh this.
Put the pusher prop on it and figure out how to start it (can't finger flip with the cage on it). Put it against a wall and watch it fall off and break. It'll probably counter-rotate as it falls off.
Now, if you're a glutton for punishment, put some wheels on the plate and see if it sticks any better. I'm thinking extending the prop away from the wall will actually cause a longer angle moment and cause it to topple off backwards.
Simply put, there is no suction to hold it to the tree, pressure will cause it to drift off to one side or the other. As an example of this, try balancing a ruler on a can. If you can, then tap the table and watch it fall off to the side.
This isn't a Monster Garage project, but I've always been the kind to try something before I get deep into the dollars. Might even be able to take one of your boat engines and use it.