My suggestion:
1) Change the focusing screen to a split-image screen. Phone OP and check stocks. They may also be able to help you install the screen.
2) Check out angle finders. They offer variable magnifications of between 1.25x/1.5x to 2.5x/2.8x. The Seagull is really clear, but tends to induce eye fatigue. The Canon Angle finder C is crystal clear and doesn't make the eye so tired. It's about 3x the price of the Seagull though.
3) Try a viewfinder magnifier. They usually magnify only the central portion at 2.3x to 2.8x. These can be gotten on ebay. I have a spare if you're interersted.
I use all three of the above as appropriate.
Generally, any magnification of 2x and above is sufficient for accurate focusing. Couple that with a split-image screen and manual focusing gets much easier, except in dark situations.