I was feeling detail-oriented when I was putting together ol' Tempus. I splurged a bit on copper tubing and compression fittings. But I made sure it had a bigger ID than the original plastic line, and I suspected the bogging issue I was having might be because the engine at WOT could use gas faster than the original line could supply it. I turned out to be right; the larger-ID of the copper line solved that problem. However, every engine is different, and some don't behave that way. That, and I've done a bit of "souping up". Where most of these little 2-smokers will get kinda close to the 150 mpg mark, I am likely only getting about 120 mpg, although when I turn the throttle, Tempus GOES! RIGHT NOW! No more bogging or any hesitation. I got me near-instant throttle response, and I'm pretty pleased with it.
But if you use copper, there'll be some customization involved. And you'll need to make a couple turns of coil in the middle to take the vibration of the engine, so the line doesn't work loose from the non-vibrating tank. I have pictures of it in my album.
Keep in mind: unless you're going for a vintage look, or something steampunk-like, regular rubber automotive-grade fuel line will work just fine, as well as some of the other stuff that the previous forum members have mentioned. Good luck.