The purpose of mounting the engine off center is to allow for side/down thrust while still keeping the crankshaft (and prop) centered in the opening of the cowl.

Check with the manufacturer FIRST to see it any side thrust/down thrust has been built in. If no side/down thrust has been built in do as stated below.

I suggest mounting the engine INITIALLY in the center.

Do not put the cowl on.

Now fly the plane.

You will most likely find that you will need to shim the motor mount for side and/or down thrust. The shimming can easily be accomplished by the use of washers between the motor mount and the firewall.

Once you have the engine shimmed to where the plane is flying properly, measure how far the crankshaft is from the center and move your motor mount that amount and permanently mount it in place. Plug the old bolt holes with some dowel epoxied in place and trimmed smooth when it is dry.

Mount your motor mount in the new location (don't forget the shims ) and test fly it. If it still flies OK, you can mount the cowl and it will be centered.