Hi ironmandreamer, welcome to Outdoorking.
We need to know a lot more about both mowers before we can comment very constructively. Please post pictures of them - if possible, several pictures of each. I'm sure our reel mower guru, Deejay, will be in a position to begin a commentary then.
We will also need to identify the engines. Most Scott Bonnars have Briggs and Stratton engines, which have a long series of numbers stamped on the cooling air cowls. The numbers are divided into three groups: the Model, Type, and Code. Please post those numbers.
In the case of the "old" mower with the "cracked engine", we need to understand what is wrong. Unless the old engine is a collector's item, and you plan to restore the mower as a show-piece, it would seem cheaper to fit a a second-hand engine, or a new modern engine. Just as an illustration, see this ebay advertisment:
http://www.ebay.com.au/itm/Genuine-...nmowers&var=&hash=item3cdcf35965
