I don't get what the problem is either. Dozens of people on this site have built the 4* 40, and I've never heard of a problem with the mount.

I'm assuming it's a standard black "plastic" mount, which should be a "40" size mount. You drill holes to match the mounting lugs on the engine; the holes should either be large enough to push a long bolt all the way through, then add a locking nut, or drilled undersized and tapped to fit the bolts you're using.

When you say the engine, "sits sideways", are you talking about a little right thrust? Because most planes have a little right thrust built in at the firewall.

Are you sure you have the mount right side up?

We need more info, pictures would be best.

By the way, not many plane kits, or ARF's, come with 'all the parts needed to finish it'. You usually have to buy stuff. Sometimes just a tank and fuel lines, sometimes wheels too, sometimes you have to buy ALL the hardware.
Dennis-