Generally two stroke engines will run at almost any angle if the carb is suitable. Chainsaws use pressurized jets to enable them to do so. The lubrication system is provided by injecting oil into the intake air flow which is then directed to the lower side of the engine and from there into the combustion chamber and this works well at any angle.

Four strokes are a bit more difficult. Wet sump engines will only work when the sump (and oil pickup) is at the lowest point. They can be made with external sumps (a common thing in motorcycles). Carburetters usually have float chambers that are sensitive to the angle but can be made to work at other angles. Fuel injection generally has little problem with the angle.

Now there are other methods to get around this. I am assuming that your problem is that you have a vertical shaft motor and need to get the output shaft axially aligned with your rear axle so you can attach 2 gears and a chain.

One solution is to find a small engine with a horizontal outputshaft. Then follow the manufacturers recommendation for max allowed install angle off of vertical. This requires buying another motor.

Another solution is to find a right angle gear box to change the output shaft without changing the engine install angle.