Remington 11 recoil spring

Don't do it. Try Jack First; not cheap but most likely to have it. Pat Laib might also.

The Browning light 12 spring will work. It is a touch longer but never seems to cause any problems.

Use the browning spring and don't worry about it. Light 12 spring. not magnum.

EBay has one listed for $14.95 and another one with end cap and friction rings for $37.95.
Gunbroker has plenty of model 11 parts listed but no recoil spring at this time.

The original Remington model 11 spring is made out of square wire, this wire wears out faster than the round wire and as the spring compresses, the corners dig into the mag tube. This causes everything to slow down and can make the gun more picky with ammunition. It is an upgrade, and recommended, to swap it out for the Browning.

Also, check your fiber wad in the back of the receiver, if it is missing or is battered, replace it. If the buffer is missing, it can cause damage to the bolt and receiver.

If the rivet for the buffer is not loose,I would leave it alone.I normally buy 11's,A5's and 720's that are "not pristine".They always need a really good scrubbing on the inside after many many decades.

I personally make my own buffers from urethane 80/90 durometer 1.125"OD / .25" thick and make a hole for the rivet to clear and a slot for the bolt tail piece that connects to the action spring.I glue them in place with black RTV.

I would clean/degrease the inside rear of the receiver really well and RTV a new felt buffer in place.

Yes,the Browning A5 recoil spring(non-magnum) works fine.

Wolff makes new action springs.