Welcome bhey. Do you have a manual? You can download it from
https://www.singer.com/support It will show many places to oil. If the old oil has varnished, it may need something else to dissolve and then oil again with sewing machine oil or Tri-Flow oil..
You said you took bobbin assembly apart, but I know from experience (mine & others) about threads being able to do a very good hiding job on the 301 and 221. Can you jiggle the hand wheel by hand and see any thing else move? Sometimes just a drop of Tri-Flow in the right place is all it takes.
I've heard tell of thread getting wrapped in the hand wheel as well. Doubt that is the problem with yours as you say the hand wheel spins if the clutch is disengaged.
Good luck - 301s are great machines.
Janey - Neat people never make the exciting discoveries I do.