I've made two of them. The first was a 1/2" dowel with the end sawed to a chisel point. The second was a 5/8" steel rod with a chisel point ground on the end. You go between the two sides of the chain, and get between the two chain sides at the tensioner, and drive the tool down to force the tensioner back in and get the needed chain slack. Use a long taper on the chisel point. Mine was about 1.5" long. The wooden tool was better for peace of mind about the chain integrity after you drive it home. Both worked ok. You need a smooth finish on the chisel point or it tends to catch on the chain.

Send that motor back. Fools have worked on it, and cursed it for all time...