I have an original line here but couldn't find it with a quick search.
I did find a pic of the glow plug in a 3600 manifold though. They are the same as the 3000.
As Sean says, there is no pressure on it.
I'm sure you could go to a hardware store and get everything you need.
About 3' of 1/8" copper tubing, a 1/8" compression fitting where it fits on the glow plug, an 1/8" Tee, about 12" of 1/8" fuel line and some hose clamps.
You could use steel line too but copper would suffice.
Cut the return line up near where it goes under the cowel and add the tee. Attach your new tubing then thread it down and along the top of the head to the glow plug.
Very, very easy task.
Hardware store will also have those little pinch open hose clamps like in the 2nd photo which I like for a neater job.