My biggest problem with my Pen FT is how dim the viewfinder is. Has anyone had any luck cleaning the viewfinder or replacing the half-silvered mirrors with regular ones to brighten the finder?
I would not touch the mirrors under any circumstance! That is a pro's job IMHO.
I would never do that myself nor would I recommend to do that yourself. The Pen F and FT cameras are difficult to repair, and there are only a few repairmen that are known for good work on these gems. A standard CLA costs between $100 and €180 and includes a C for the finder (the C one that comes in a CLA).
The well-known ones are probably these:
And there is one guy who helps those who want a black Pen:
The half silvered mirror is required for the meter to work. An FT with a "regular" mirror is called a Pen FV.