Welcome to the world motorized bicycles!!! Aluminum frames are not recommended because they tend to crack due to the vibrations from the engine, but I think it all depends on the type Aluminum alloy. For engine mounts, you can get it from Manic Mechanic. I got rid of my derailers and start using internal gear hub, so I can use BMX chain that are tough enough to handle the power and my weight. I'm using the shifter kit with rear disc brakes.
Also, you will still need to do a lot of things to the engine before installation. You need to a number of things on the engine. Most of us opens/ dismantles the engine before installation. Enjoy your build!!!