Don't bother with those Knobs - if your replace them you will either accidentally move one or someone will fiddle with them thinking they are something else & it will never start or run properly.
They are the mixture controls - which may be useful to have on the front panel if you regularly take a dinghy from the sea to some Alpine lake 1.5 miles up a mountain, but the best thing you can do with them is set, lock & put a grommet over the hole to make it look pretty. My '72 vintage 25 has(had) them too. I used one hole for the deadman switch!
Unless of course you want to return it to "as built" condidion...
If no reply here, try Mitchells In Renfrew, they'll post that sort of thing out. DA in Dundee I think they are Yam dealers but I have gotten all sorts of bits from them.