If the nipple is brass or a metal you can solder you can just use telescoping brass tube to fill the nipple until it's the size you want. The K&S stuff works well for things like this. I've done it for a couple things that I don't remember but it wasn't for an engine. Basically I just coated all the pieces with non-corrosive flux, then slid all the brass together and used micro torch to flow solder in. It will flow between the tubes. It doesn't need to be air-tight because they go inside the nipple and when the fuel tube goes on it's all contained.
Edit: Of course if you have some brass rod you could just solder it in place and drill it to whatever size you want.