Dennis Waldron is an authority on Postwar. He occasionally posts here on the Forum. He bought a lot of hardware and documents from the original Lionel factory, including a machine which can supposedly restore magne-traction. If he's willing to do it, that's the only way I know of. I'm pretty sure the magnets are embedded in the chassis and are not easily replaceable.
I'm curious, which locos are you comparing it to? The most similar would be 8600 Empire State, 8702 Southern Crescent, 8801 Blue Comet, and the 8210 Joshua Lionel Cowen Hudson. I'm guessing that an original postwar 2046 in good condition would probably be stronger. I've heard some folks say to store your loco on a piece of tubular track because it acts as a "keeper" to preserve the strength of the permanent magnets. If this has been sitting in its box for 39 years, some of the flux may be lost.
The Berkshires (736 and MPC clones) used a different placement of magnets, and I understand that they have a stronger grip on the rails, especially the last Berk made by MPC, the 8615 L&N / JC Penney, which had extra magnets added. A good one of those sticks like glue!
Also make sure you are testing all of them with the same piece of track, because the ferrous content in the tinplate rail varied quite a bit over the years. Again the old 1950s O probably had the highest iron content and best grip. Honestly the condition of the wheel tread, whether the wheels and axles run "true" or eccentric, etc., probably impacts traction more than the strength of the magnetism. My $.02.
