Grumpy, there are definately signs of orange on the engine cowl, you can see it in the photos I posted earlier. I guess this is proof that Scott Bonnar were still producing orange engines in 1981 as discussed in the post you pointed to. If this is the case then it is possible that this is the original engine for this mower.
With regards to the engine, I am not confident that it is in a servicable condition. I still haven't removed the engine but I was having a look at it last night and when I turn the crankshaft forwards there is a horrible noise coming from inside the engine. It sounds like loose metal parts! Also, I don't seem to be able to turn the crankshaft backwards. I will let you know more when I have the engine removed.
Steve
