You probably have some surface rust in the bores, after that long sitting, & your piston rings are probably gummed up, stuck in old oil that has hardened & dried to some extent. This is not terminal
Have you done a compression test on the engine?
My present 7 had sat for over 2 years, with a shot clutch. It's compression was down about 40 PSI on 2 cylinders & a couple of valves were sticking not quite shut, & had the problems I expect your engine has. After a couple of quick oil changes using STP & a quality 30/70 oil, in about 200Km, it was running quite well.
Another couple of thousand kilometers, with the valve clearances redone twice, [some did hammer in a bit, which means the seats were a bit rusty too], & it was running beautifully. That was 60,000 Km, & 10 years ago. It has only got better since then.
Yours are probably problems that can be treated with STP engine oil treatment. It is excellent at helping polish rust off bores, freeing up gummy rings & lifting carbon of things like exhaust valves.
With your engine I'd add a bottle of STP to the oil in there, run it for 50 to 200Km, depending on how quickly the oil got dirty, then do another change, with STP for a further 500Km.
Provided it is properly tuned, with the right needles in the carbs, & valve clearances OK, it should be getting as good as it is going to by then.
If successful you've saved a fortune. If not you have wasted 40 quid. Good odds in my book.
Hasbeen