Hi heady,
Firstly welcome to the ODK Forums.
Sorry that I can't be the bearer of any good news but some years ago there was a batch of around 150 metal replacement units made and they sold out within weeks. There were never any more made.
Sadly it's just a matter of time until yours lets go and thus why these have developed such a poor reputation through out the Lawnmower industry.
When it goes bang it will revert to a push mower, or even better a donor engine for a Model 45 with a crook power plant.
As previously said, sorry to be the bearer of bad news.
Cheers,
BB.
PS. Hope you didn't pay any more than $50 as basically you've bought an engine. Sorry that this sounds brutal but that's how it is.