That brake is garbage and the distance between his wheel and fork crown will be no different than the distance of a 20" or 26" wheel, with the proper fork. The wheel size is different but the gap will be the same regardless.

OP don't get that ****** caliper, get a side pull for a BMX bike. It'll fit. Bikes that use, or are side pull capable typically use the same size caliper. The only exclusions to this are road bikes that use a smaller one for the narrower wheel and smaller gap between the wheel and fork crown. Go get a BMX side pull, it fits those, it fits my laJolla, it'll fit your Onyx, it'll fit one of those 30" Schwinn cruisers from the mid-00s.