Almost all the early cars had those vents. Later cars with the straight dash went to the more common rectangular vents found on lots of cars. If I don't have any luck finding the donor vehicle, try Miata or Ford flap style vents. The hose connection is smaller, but the surround is large enough to cover the overall hole itself. Some clever gluing with some pvc pipe can make a snug fit. The rear lights (bi-color) are indeed Bedford. I've got a couple spares here and am always looking for more. They show from time to time on eBay.uk . I don't have the light bulb sockets for them though, but I think any of the replacement sockets can be made to work. They're just simple plug and twist - check your local FLAPS for those.
Never assume anything on kit cars that are 30 years old. As you've found, accessory parts are impossible to find. Many of us have small stockpiles of stuff we've come across over the years, so don't hesitate to ask. Remember, too, that each car was usually built by it's original owner, so what's in your car may not be in someone elses. Very few cars were "factory", but they are out there!