Just picked up an 03 ecotec cavy. Little back story, the timing was put in 180 degrees out bent valves etc. The timing was redone correctly, head sent and machined, flow tested and all put back together. Car ran like a champ for 3 days then just stalled out. The guy i bought it from said it needs rings now and rod bearings. The car will idle fine drive fine if kept under 3000 rpms, after that it bucks and struggles to go. There are no knocks. The oil it pitch black but has no coolant in it or anything. I changed the plugs that were low mileage but very black and had some oil on them. My question is does this sound like rings?

Not sure if im correct or not but could you have just foul out your plugs?

Oil on plugs and in cylinder is piston rings or damaged piston. When it was 180 out it might have done hair line cracks In pistons that they didn't see but when under pressure leak. I would pressure test each cylinder and see how far off compression is from one cylinder to the other. if way off bad rings or head gasket. I would just do a rebuild only way to 100% know what's wrong in engine.You can get a rebuild kit with pistons for around 500$. Hope this helps