Could you get a 90 degree speedo drive, like you can fit a Toyota one with the MX5 gearbox?
Finding out the Scorpio final drive ratio etc. shouldn't be an issue to figure out how different the Scorpio and Sierra specs are and whether you could swap from one to the other with no further work.
There are companies out there who can recalibrate speedos (the one I know of is Speedy Cables in Swansea). They charge £65 + VAT and up.
I think the cheapest and the easiest thing to do would be to get a 90 degree adapter. It could be pretty cheap if you can get it off a scrap car.