i can't make it out real clearly, but is that the crack that looks to run nearly all the way around the outside of the bolt holes? looking at the condition of the rest of the wheel it's hard to believe that it's been stressed enough to crack that badly without any catastrophic damage elsewhere to the rim - especially on a front wheel. forgive me for asking, but it does go into the metal and not just in the paint/powdercoating?
if it's cracked on one side, i'd just have the crack cleaned and welded. grind as necessary to put the lugs back on. if it looks to be cracked nearly through, possibly reinforce it by welding another plate to the wheel. check and see if anyone around has a cnc burning table that can cut you a precision center that can be welded in. today's plasma cutters, when coupled with a cnc controller can cut good quality bolt holes - especially for a low speed application. i've seen people on ebay making cnc parts for around $1 a pound. even if you used 1/4" plate, you'd only be in about $10-15 plus shipping for a piece that is a perfect fit into the center of that wheel.