Getting a plastic company to re-manufacture it will cost many thousands of dollars.

You aren't asking for re-manufacture of an existing product, you are asking for a whole new item to be tooled up which is a mixture of two existing products.

If you really want a ramped agitator for your Simpson, look at other existing ramped agitators that were used on washing machines that were sold here in Australia - such as Westinghouse top loaders from the 1970s and Hoovers from 1970s and 1980s, see if any of them could be adapted for your machine. Hoover had two versions, the long shaft which joined the agitator above the water (spline at top of agitator through a brass block), and the later short shaft which was below water, the agitator presded directly on the the shaft just like Simpson autos of the time did. I don't know if that spline matched the Simpson one or not, they look the same but are probably different???

I don't think you have any chance of doing what you are suggesting, and should probably look for other projects, but adapting an existing Australian agitator to fit the Simpson spline will be much easier than commissioning a whole new agitator. If the splines are similar but not exactly the same, and the agitator otherwise fits, you MIGHT have some luck with using epoxy putty to create a new spline in your chosen agitator. But that might chew out in a few minutes, too. It would be a "try it and see" situation, no guarantees.

There have been many discussions on this website over the years about getting obsolete parts re-manufactured, and even when there is an existing part to copy (which you don't have) the cost for tooling is thousands of dollars. It's an expensive business.

Maybe you could start a search for a wringer washer with a spiral ramp agitator? Westinghouse made wringer washers in Australia, they used spiral ramp agitators in their automatics so they might have made wringers with them too?? I don't know if they did or didn't, it's homework for you to find out.

That spiral ramp agitator fits OK in the washtub, doesn't it? Looks OK to me?

It would be cheaper and easier to get a new, larger drive block made up to adapt the Simpson agitator shaft to the new agitator's hex drive socket.

If the spiral agitator is too big to fit, what about a UK Hotpoint top loader - didn't they use a smaller spiral ramp agitator? They were made in New Zealand under license by Fisher and Paykel too, though I don't remember the model name. They were made by F&P before the ECS / Gentle Annie of their own design. So you might be able to source an agitator from NZ?? (Still no guarantee the drive block will match.)