Like the others said, just get a long chain, like 130 links, and cut it to length. Mount the rear wheel, sliding it in closest to the engine. Wrap the chain around the prockets, and pull it tight, mark the link your going to cut off with a sharpie. Any new chain will stretch a little. Higher quality chains will stretch less.
X-Ring is the best, then O-Ring. You can't really go wrong with a high quality name brand chain, RK, EK, DID, BikeMaster, REGINA.... Yeah, you'll spend about $100 or more, but it's worth it. Most modern high quality chains are Rivit master link type, either barrow a chain rivit tool, or get one from MikesXS.