It‘s not the worst mismatch I have seen.
I see metal at the oil pump pickup, so it must be a PRO model someone tossed an 026 tank on.
The jug looks AM to me, the top center boss is always machined flat on the 44mm decomp jugs. I have seen that boss rounded, but generally only on non decomp 44.7 jugs which would only be original on late 260’s, not 026.
If you got it for nothing, it’s not gonna cost that much to get it going. For the tank, you can buy the wider filter with the proper compensating hole in it. You’ll need to modify the tall black tank vent to make that filter work, it’s not the end of the world. There should be a red flapper valve under that tank vent opening. You should be able to see a tit popping through from the outside. Some have even used a piece or tubing and popped a small echo mushroom tank vent into the factory 026 tank hole. The flapper valve does most of the sealing for the vent operation. You could literally add any type of breathable fuel resistant material into that hole to keep debris out and call it a day.
No sure on that carb though. They usually slide right into the tank with some sort of grommet. Not sure why the tank was molested with a grinder to make it fit. I think that may be a later Zama carb for the MS260, or an AM variant.
So, in the end my bet is that you have an 026 PRO case with an AM top end. They added an older 026 tank to it, put some other carb on it, and thought up that ingenious wood screw to hold the rear cover on. The airfilter screws on the wide one sit flush, those older ones stand-off and the older rear locks between them.
