I'm not familiar with the engine mounting of this kit, but normally you simply loosen (NOT remove) the engine mount screws, place a piece of cardboard between the two gears, move the motor slightly until the card (Cornflakes, etc) is firm but the gears can move, ensure the alignment is STRAIGHT (very important for gear longevity), tighten 2 opposing screws to hold in position and turn the gears to remove the card.
You should have a little slack (perhaps 1mm) but the meshing must be consistent.
Before you fully tighten the motor screws, check one complete rotation of the spur and pinion, to see there's no binding from ovalling (some gears are not as 'round' as you may hope!!).
NOW, tighten the mount screws and repeat the check.
Sorted.
Al.