Hi. I feel like this is an easy answer but one that eludes me.

Bought a 1994 K75S mid-restoration from prior owner. He had swapped out the fork with a 2012 Ninja ZX6R fork & Cognito Moto Triple Tree/Stem. I'd like to use the original 3-spoke K75S wheel with it, but it appears I'd need spacers to fill the gap between the fork and wheel.

How can I identify the correct size spacers to buy? Apologies in advance if this is an extremely simple question. I don't have much background on forks, wheels, etc.

The spacers just ensure that the wheel sits centrally in the fork. In principle the 'important' spacer is the one on the LH side (viewed as you sit on the bike) This spacer is of the correct width to ensure that the wheel is aligned centrally. The second, RH spacer isn't a spacer at all. It is just a washer that sits up against the wider part of the spindle and draws the wheel up against the LH fork leg as the spindle end bolt is tightened. The RH fork leg then remains free to float axially on the wider end of the spindle and this is why it is important to bounce the forks a few times during assembly to ensure that it has 'floated' to the correct place before you tighten the pinch bolts on the bottom of the fork legs.
Do you know if the OE spindle is being used? If so, you might be able to re-use the original spacers if you have them, but with the LH spacer altered in length to give the correct alignment of the wheel. Otherwise, if you have to source new spacers you will have to ensure that the wheel side of the spacers is correctly profiled to match the dimensions of the wheel hub and bearing.

Befriend a machinist and ply him/her with good whikkey. It's unlikely you'll be walking into a shop and plucking the correct dimension collars or spacers from a neat tray marked "Choose yer own axle spacers".

You will find the original spacers you need here:  

www.realoem.com/bmw/enUS/partgrp?id=0563-EUR-05-1991-K569-BMW-K_75_S_0563,0572_&mg=36 

The next question is, do you have the original forks and axle to go with them?  

Motorworks have them as used parts and still have new stock of the smaller spacer.  These are stock items across R & K series of the same vintage, so relatively common.  They are 8mm and 27mm as stated in realoem and the axle OD is 25mm.