Re: bmx w/Yamaha 100
The difference is that motorcycle tires are measured at the rim and bicycle tires are measured at the outer circumference, and very loosely measured at that... that's why you'll need 16" motorcycle tires instead of 20" bicycle tires.
A set of 16x2.5 or 3.0 tires would look great on that bike and have a lot better traction not to mention the ability to withstand the speed that thing's gotta be capable of... I'm guessing it can do at least 60mph if not better so if it was me I'd definitely be putting some safe rubber on those wheels... I'd also want to re lace at least the rear wheel with the thickest spokes that you can make fit. A set of 12 or even 11 gauge spokes would be a lot stronger than the typical 14 gauge that could possibly rip out from several hard take offs...