I'm a professional mechanic at Ford/Lincoln/Mercury, and after 12 years, there's a good $50,000 in tools between work and home. Needless to say, it's unbearable working without the proper tool for the job. For the bicycle tool box, which is filled, is uneccessarily so. 95% of needed tools are also carried with me on the ride (although at the small sacrafice for space, for example a folding allen set vs a set a L-bent-ball-end-allens).
So the 95% needed for most things (read this as MINIMAL neccessary, and NOT counting fabrication tools, which motorbiking is heavily involving!) are:
Two bicycle chainbreakers, one with locating "tabs" removed to work on wider motor chain.
Long-handled ball-end metric allen set.
8-15mm combo wrenches.
8-15mm deep socket set 1/4" drive.
#1 and #2 phillips screwdriver.
1/4" flathead screwdriver.
Small flathead screwdriver with pocket clip, my most used tool by far at work!
Set of teeny-tiny scredrivers.
Tire-removing levers(with a notch or hook to to hang onto spoke after prying so you can free your hand).
Tire pump/patch kit. (floor pump or CO2 cartridge type for the ride itself).
Small(6-8") channelock pliers.
The smallest gasket scraper you can find (or make one from spare kitchen table knives cut off somewhat near the handle, one straight, and two at left/right angles, and sharpened to a perfectly smooth, burr-free edge. When using a grinding/cutting wheel, use water frequently to reduce temper loss.)
Loctite, medium and strong (stud mount).
Wheel bearing grease.
Acid brush for grease application.
Motorcycle (non-flinging) chain lube.
Brake cleaner for gasket surface/carb cleaning.
Simple green for exterior cleaning.
Fast Orange handcleaner.
LOTS of rags/shop towels.
Bike stand (actually, my bike stand has been rendered useless with the gas tank etc now in the way of the frame clamp).
Cooler and beer bottle opener.
And most important=good lighting. For countless years of experience, I've found the best light source is a headlamp, like found in camping stores. Online search for a 1 watt LED headlamp at least, with adjustable angle tilt, you'll never go back to another light source again! Overhead/floor lighting, no matter how bright, casts shadows right where you need the light!
This is far from a complete toolbox, BUT, if a someone is starting from the beginning, this kit will do the vast majority of mechanical duties needed.