Pat:

Two ways to go.

1) If he can get to the back of the firewall after the plane is built, no further attention is needed at this time. Build it, wait until he gets the engine, and then mount it.

2) If the back of the firewall will not be accessible, decide on the engine, buy the appropriate mount without the engine, bolt the mount on and finish the plane.

Actually, there's a third way - set the kit aside until he gets the engine.

But if he wants to build now he has the two options.

HTH.

Bill.