You can make up some wedges from either aluminium or a good hardwood like maple. Adjust the angle to remove the upthrust and add a further 2 or 3 degrees downthrust. If you're using a radial mount make a single wedge out of 1/8 or 3/16 ply to fit between the firewall and the mount. Some folks try to just use shims but that often distorts the firewall and the engine mount if it's plastic . If you DO want to try just shimming it be sure the shims are right down by the screws. You may also need longer screws to fit.

The shims are a good idea even if you only use them to check on the angle required before you make up the better option of the wedge. Just use ply or hard balsa bits and snug the screws without tightening them for measuring. When happy make up something more robust that is permanent.