G'day kenny_t,
As AVB says, chasing OEM parts will be difficult - some are available from US suppliers, but postage is a killer there.
FWIW, you can download an IPL from
https://www.leonschainsawpartsandrepair.com/parts-diagrams.htmlYour best bet might be to try a 'universal fit' string head, that comes with several shaft adaptors. A search on eBay Oz for 'speed feed trimmer head' will find a few - the 3.75" size is correct for an 18cc trimmer.
You will first have to determine the diameter, pitch and hand [i.e. LH/RH] of the shaft thread, though.
If it is a left hand thread, it will likely be metric, and these are the easiest to find heads for.