Ok, @Stan Galat, I think I massively oversold myself with 'I fabricated..', more like 'I made' - there's not a lot to making a woodruff key, just a solid piece of metal in a particular shape. All I needed was a larger piece of metal, a hacksaw and a file! I only wish I knew my way around a MIG/TIG welder. I will have to call my brother (who's only 12 miles away) or friend to see if they can add on a few inches to the oil filler pipe.
Guys, thanks for the tip about breather hose size. I must admit I didn't think of a hose enlarging and sagging over time. The fittings are all 5/8" OD so I'll get a hose with 5/8" ID.
And as for the cone, thanks for reminding me to consider the easiest option, Danny. Obvious case of not seeing the wood for the trees.. A quick search on UK eBay and there's one in good secondhand condition for £100 ($137) including delivery. Sorted! And a LOT less hassle. Fingers crossed it will fit once I get it next week.