Researching this, I thought it might be helpful to list part numbers / alternatives on here for the help & assistance of others. (Many thanks to Ruffus for his helpful contributions so far.) I'll come back and update this as I go along.
Item: OEM Part Number: Alternative to consider:
Oil Filter 1541A-LEA7-E00 HiFlo HF566
O ring seal 91303-LEA7-E00 eBay Search for "57x52,5x2.5 oil seal" or similar where 57 is the OD, 52 the ID & 2.5 the CS
Drain Plug Crush Washer 90521-GFY6-C00 eBay search "MOTORCYCLE M14 SUMP PLUG COPPER DRAIN PLUG WASHER 14mm" or similar
