Here is the fuel tank on my version of my just completed Rupnow vertical
The tank is all stainless steel 28mm dia tube 70mm long with a 2 mm wall
The ends are a light press fit with a smear of green loctite.
congratulations sir on getting your Webster engine running.
I had the same problem with the head rocking back and fourth I fixed it by putting two extra 4mm screws in the bottom, it worked for me.
I used a set of Lucas points and condenser £2.95 on eBay and the coil from a scrap yard as you can see in the pics the coil and condenser are mounted underneath inside the box.
if you want to build a i/c engine then the Webster is a good place to start. There are several build threads on the making of this engine you can find the plans here Free Plan: Webster Engine Work's 4-Cycle Gas Engine | MachinistBlog.com