Not to start an arguement, but I aggree with Grumpy. However at the end of the day it really comes down to what you want to use the mower for and how far you want to go with the restoration.
I myself have a Scott Bonnar 45 painted up in Rover Red, with a few late model Rover bits including the catcher. But at the end of the day I wanted a reliable reel mower that presents well. The purist will soon tell that it is not a Rover 45
As for repairs to your chassis, I certainly would not be Arc welding it, as the inherant distortion or risk of poor penetration could be future problems. If it was mine I would be having a go at repairing that Rail, but leave the roller and sold plate attached to chassis while welding to gain as much rigidity in the chassis whilst welding it. Clamp a scrap piece of solid material to the rail, that is the same length to hold it straight. Then I would fit a piece on the underside of the old rail. The piece should not be any thicker than twice the thickness of the rail as you will not get a strong weld and require to much heat, causing distortion. The piece will only need to be stitch welded, but allow time for the rail to cool between each weld and work from the middle out. Tou can then weld the crack on the outside and grind it back.
Overall you may get a slight twist on the chassis and you would have to way up if this is worth the risk, or how critical you will be on the finish the mower gives. I have seen new 17 inch Rover Chassis on Turf Machinery website, but dont know if you will get a 20 inch. Plus the chance of getting one 2nd hand in WA is quite slim.
My Briggs has a deep tank and was the original Orange engine on my 45.
