Hopefully the motor mounting holes are slotted, and hopefully there is a little bit of wiggle room left.
Slacken the 4 x bolts off a touch, and use some blocks of wood to create a 'wedge' between the motor and compressor - in order to push them apart with a lever - crowbar, pry-bar, garden spade etc....lol Basically if the compressor doesn't have jacking screws, then you just have to improvise with what you have.
The belt needs about 1/2 inch deflection measured at the centre of the two pullies - just like a car fan belt.
Tighten the bolts up, and check for squealing, slipping.....you should be fine. It will usually only slip under severe loading.
.....and for the flopping up & down......try taking two "Mycoxaphlopins' before dinner. Take with plenty of water, or else you'll end up with a stiff neck.