I have a 1 that works great and it is doubtful you'd find much meaningful difference between the versions. I'm going to go with the CLA recommendation. If the camera hasn't been CLA'd then it is pretty much certain that the prism foam has degraded and damaged the prism silvering. That is really not a big problem as the polished glass reflects well by itself. Polish the glass and feather the edge of the silvering with a mild abrasive such as toothpaste, and you won't even notice the damage. I believe having seen that an OM-10 prism will fit, but don't recall exactly. Either way, once it is fixed up it will be a joy for many years.
I find the OM-1 smoother and quieter than the OM-1N. I have 5 OM-1's and 2 OM-1N's to compare.
I like the OM-1 because it is more pure in appearance, especially around the eyepiece and hot shoe mounting area. (but that's just my obsessive compulsion order showing)
One other thing. The Black OM-1 has an all-black meter switch and the Black OM-1N has a white stripe in the switch which I find annoying. (compulsion...)
OM-1n without hotshoe + 50mm f/1.4 > 1.1m lens + metal hood.
Loads of film + cheaky hand-held lightmeter + shoulder bag.
*Lens choices may vary. My perference is 35mm f/2.8 for street photos or 28mm if you shoot buildings + monuments + ruins(which I did in Italy)
Had an OM1n for a long time. Have an OM1 now. No noticeable difference. The most important is to get it checked and the meter adjusted as 625 batteries are no more available.
There were a number of incremental changes in the design and construction of the OM-1 over its production life. All of those changes were incorporated in the OM-1n.
I think you will find that repair people like John Hermanson will recommend the OM-1n.
In addition, the OM-1n shares the Hot Shoe 4 with the OM-2n, although Hot Shoe 4's in undamaged condition can be hard to find.