Whoa.. is that a section of piston ring? If it is... the only way a part like that could make it past the piston is if parts of that piston are gone too.. Either way, it's gonna be some work. Sorry to see that A-J. Good luck.
If you broke a piston bad enough for a piece of a piston ring that big to make its way to the sump, I would expect to see more pieces of piston, also. Did the bike suddenly belch great volumes of oil smoke? It might be that the piece of ring, as well as the sealant found in the sump were left over from the last "rebuild", and the new loss of oil pressure was due to a plugged oil screen???
I really couldnt say where exactly it came from. I know it started feeling "not right" then I lost pressure. No smoking though. The bike didnt seize. Just starting making the wrong kind of noise, no clanking or clacking or anything, just didnt sound right. I cant really say what kind, just that it was like metal on metal. No smoke. And it did kick over after cooling a bit. But I only had it on for a second or two. The bike still shifts through gears alright with it not running. I cant really turn the back wheel with it in 5th anymore. It will move but it is stiff, and will move for a bit then its stiff again. It might be nice if I knew the right adjectives to use here...
I dont understand how the piston ring got through the screen. I should have had a picture of what the screen looked like. Stalactites comes to mind. I thought maybe an oil passageway might be blocked and that caused it to over heat. and blammo. But again, I know how to change my oil, clutch cable, rebuild my forks, a little wiring, various miscellany. This has been the extent of my first hand motorcycle schooling.
Is there a test I can do to find out if this is actually a piston ring portion or the other part youre mentioning? I threw it away, so I couldnt measure the curve of it. The guys at the co op were fairly certain when they saw it that it was piston ring.
Piston ring and bearing retainers look very similar, except bearing retainers are half-round parts where rings are fully round (until broken...).
It appears that someone has assembled the engine in the past with 'way too much gasket sealant, hence the collection on your screen and the globs in the pan. The metal chunk you have there has been properly captured in the pan, so it didn't go through the engine, and the screen on the bottom of the oil pump has been collecting thegoo and preventing that chunk from futher circulating, all part of the genius of the design in this dry-sump engine.
This chunk is the clue to what happened, but it is hard to tell what that was until the cases are opened for a look-see. I was about to ask if it had a ring mark on the end, but if you don't have it now, I can't ask. Retainers have no marks: piston rings have a tiny engraving mark on the squared end (which is the top-side of the ring when assembling).
If it was a piston ring from the current top end, the engine would not have run at all from such a catastrophic failure, as it would be from the upper rings and the whole side of the piston would have to be gone to allow it to fall into the case. My estimation at this point is that this event happened before you got the bike, and this is debris left over from a top-end rebuild, with too much sealant applied. The symptoms you describe sound more like a seized camshaft than anything else I can think of, which could come from plugged oil jets in the head: these could have been plugged from too much sealant applied to the base gasket under the cylinders, which then would have moved up the 2 oil passages to the head and slowed down the oil flow to the top end. It would take a little while then to tear up the cam and cam bearings, which seems to be the chronology.
All these musings won't get you back on the road, however.
In your situation, looking around for a used engine would be the least expensive path to running again, followed by parting out the engine you have for cost recovery. On eBay or CL, you can make $35-$50 for each engine cover, and if the cases are good, someone on the forum might like to buy them to replace some that were crashed with a thrown chain (I can think of several candidates!). The sprocket cover is also a hot item on eBay, I've noticed (that's the one with 2 screws that covers the sprocket at the rear, chain side).
Rebuilding: the costs for taking it apart begin with the gasket kit, about $100. But...I suspect that taking the engine out, followed by removing the cam cover and inspecting the cam and bearings, would be all the further you would have to tear down. In that case, all you would need would be a new cam cover gasket instead, plus a used cam and cam bearings, possibly a rocker or two. I know this forum can come up with those kinds of pieces on the cheap, as I was in a similar situation 2 years ago and when I asked, I ended up with all the parts for nothing but the postage (I was unemployed at the time). This is a great community, all of whom would like to see your smiling face collecting bugs again as soon as possible. So, since the bike is a boat anchor at the moment, maybe taking the engine out for a peek under that cam cover is the wisest first step?
