Weld it and be prepared to clean out any over penetration in the lathe. If it's a big lathe you can mount it with the barrel up the spindle bore. Four jaw will be essential for perfect concentricity. If the machine isn't big enough , unsolder and detach the cylinder and use the fixed steady.
Or get hold of that replacement cylinder. It's not a five minute job whichever way you go at it.

Atb Mark