Re: Mounting rear wheel/sprocket first?
There is no reason why you can't install the entire kit, less the rear wheel sprocket.
I usually mount the sprocket last. Using the kit supplied method of mounting the engine, you don't have any side-to-side adjustment. There are options for aligning the wheel sprocket with the engine.
You don't want to install the sprocket and then discover that it does not align with the engine when you install the chain.