PS,
The paddleducks engine was designed so that you can use whatever you have available around the place, so don't worry if you can't get exact sizes and thicknesses, if you think it will be strong enough, use it. So if you don't have anything large enough to make the two cylinders in one piece, look to see if you have materials to make it in two smaller bits. Nothing is ever written in stone.
Whenever I build from just plans, I don't go ordering anything special, I just grab what is available and make do, 99 times out of 100, you will find that engine will run just as well. In fact, everyone should look at making engines from plans slightly differently, then you end up with a unique power plant.
John