Re: 1st timer
The chain tensioner shouldn't be used to guide the chain left or right. You'll end up with a chain that jumps off and breaks spokes.
The rear sprocket teeth are on a kind of beveled edge, so flipping the rear sprocket will shift the chain left or right accordingly. Still, just a little bit.
If you mount the engine straight, it'll line up closely enough. Make sure you have enough play in the chain. A too tight chain won't be forgiving, a too loose chain will slap all over. Enough slack makes up for a little misalignment without falling off.