Don't get discouraged.

Insure the engine you have chosen will provide adequate power for the plane.

Keep the propeller in the same location, you may need to adjust the location of the firewall in order to do this. Build the firewall strong enough to withstand the additional vibration of the engine.

Fuelproof the firewall and be sure to seal the fuselage to keep fuel/oil out.

Decide where you want the exhaust oil to cover the plane and orient the engine accordingly.

Think about COG as you build and place components accordingly.

You'll want to keep the fuel tank as close to the engine as possible. As already mentioned decide if you'll use a 2 or 3 line system for your fuel tank and how you'll refuel.

That's all I can think of for now...
Glenn