Steve, the engine being of unknown but greater age than expected, has to be considered in combination with the mower being painted red. As far as I know, only the Rover 45s were red, the Scott Bonnar ones were green. We have to consider the possibility that rather than being a Rover 45 retrofitted with an engine older than the mower, we could have a Scott Bonnar mower repainted to disguise it as a Rover. That does not seem to be an unusual thing to happen. However note that most collectors seem to prefer the SB mowers to the Rovers: the main feature of the Rovers that people seem to have liked is the colour, not the build quality.
I think you need to find out whether the mower is a Rover or a disguised SB. Deejay can ask you some questions that will make it clear which it is. Beyond that, fixing the broken chassis problem is fairly difficult, because the chances are the whole frame has twisted like a corkscrew, and restoring it to perfect alignment will now be difficult. Arc welding alone will cause a fair amount of distortion - I understand the SB mowers were spot welded rather than arc welded, which made it possible for them to leave the factory with straight frames. I don't know how the Rovers were welded. The issue is that SB, and Rover, used the soleplate (the casting that holds the bedknife) as the primary alignment reference when assembling the mower. It was clamped in a large jig, the soleplate was mounted, and the frame was spot-welded while in the jig. Yours now probably (almost inevitably in fact) has one sideplate rotated relative to the other, with only the soleplate's torsional rigidity limiting how far out of alignment it is. This matters for a couple of reasons. One is that a twisted soleplate is likely to give a poor fit to the reel. The other is that unfortunately SB/Rover did not provide a way to re-align the front roller to the soleplate once the original alignment, provided by the assembly jig, has been lost. There is an alignment adjustment for the rear roller, but not the front one.
Deejay recommended that you strip the mower to the basic frame and have it aligned and repaired by a tradesman welder with proper facilities. I agree with him. The problem is to duplicate the SB assembly jig, and then arc weld the frame without distorting it. I would not like to attempt either task myself.
