I have another motorcycle cake to do next weekend. Last time to color the gas tank I used luster dust in vodka and brushed it on. PITA! Didn't care for all the brush marks and was super hard not to get it on the rest of the bike. I have since invested in an airbrush. My question: Airbrush it on first and then let dry slightly before I put it on the bike?? I'm afraid it will crack then. If I try to airbrush it after it's on the bike will the overspray make a mess??

You could hold a piece of paper blocking whatever you aren't airbrushing to take care of the overspray. I find that works very well. That is what I usually do. I also for some things spray and let dry like you were thinking of doing.