All three years are different as you probably know.
1990 has conventional forks, a shock design from the mid 80's, and round mirrors.
1991 has USD showa forks and square mirrors, but the engine is the same except for the placement of a couple of sensors.
1992 has the Terblanche restyle like the 888 and a slightly hotter motor.
In my experience the engine is not too bad to work on, although going by the maintenance schedule of the time you will have the bike apart more than together. Valve adjustments perscribed every 3000km!
Some parts these days are becoming very hard to find. Rear tail lights and the seat cowl being two. I don't think Corbin makes a seat for it now, but it is a big improvement of the stock prostate pounder.
There is another forum online dedicated to the 851 & 888. Not sure I can post the link on Ducati.ms
Marvin is the man when it comes to the 851/888 series bikes! Check out the web page as he explains above and you'll learn everything you need to know about the bike!If it's a 92' the heads, collets, etc. are the same as every other 4 valve Desmoquattro. The 92' 851 and 93'-94 888's all have the same valve size as the 748 and 916. Cams are the same as 916/996 unless it's a SP model. (Except that one of the cams has the provision for a mechanical tachometer drive)
The earlier ones had smaller valves and the valve collets were a little different but the new style collets work.
Maint. is no different than a 748/916/996.
Go for it! What year is it?
Tons of info. here. Ducati 851 and 888 Forum
There are some differences between the pre-92 and the later bikes but not a lot. All that info. is on our 851/888 Forum