MMC do have a limited stock of many engine spares and have had a full engine gasket set produced.
There are a few engine parts stockpiled at owners and a few sources such as Krazy Horse and M3WServices in the UK and France. These are not generally for open sale but if you do tear down your engine and need specific parts they may well be possible to get hold of.
Alternatively over the years there's been lots of chat about retrofitting a Harley engine so you could try that. One such thread is
Engine Conversions
