It isn't surprising that the mower is an SB45 dressed up as a Rover 45, and that is no bad thing - the SBs were more collectible and are reputedly better-built. There may be cause to be irritated with the person who dressed this particular piece of mutton as lamb, because in the process he or she seems to have thrown away the mower's model identification plate and the Briggs engine's original cowl. Its collectibility is severely damaged by those events, plus there is now probably no way to work out when it was built.
The main immediate issues are getting the broken frame rail, and its downstream effects, fixed. Probably most dealers would say that there is no problem either with welding up the broken rail or arc welding in a replacement rail. If it was carefully done the resulting misalignment of the chassis might only amount to a few millimetres when measured with the usual alignment checking tool, and that might be barely visible on the mowed lawn it produced. There is some practical merit in that argument, but we are talking about a collectible mower here, and Outdoorking generally recommends good workshop practice. Good workshop practice would repair the mower to something close to its original quality when it left the Scott Bonnar factory.
So far as the engine is concerned, we don't know what it is, or whether it was the original one fitted to the mower. The only reason to think it might not be original, is the deep fuel tank. Has anyone seen a genuine SB45 with a deep fuel tank on its Briggs engine? If anyone has, this becomes a non-issue, and you can expect that it is an 80202 as advertised, but not one made in 1981. We also don't know whether the crankshaft has been damaged by the experience of running for an unknown period with a broken frame rail. My view of that is that if the crankshaft keyway isn't damaged, and the crankshaft extension (Power Take-Off in Briggs-speak) runs true, there is no point in worrying about it, it will probably last longer than I will, perhaps much longer.
