It is true that the best thing to do is have a Hasselblad experienced repair guy check it, especially for mirror alignment in an old body. If your main problem is that the screen is dirty you could take care to remove the screen and count the turns on the screws as you take them out and note which way up the two screens are. Wash the screen with a little detergent, don't use anything stronger than alcohol if you wanted to use a spirit solvent, you may lose the black crosses. Finally put it all back and using a loupe or magnifier to check the focus on the screen at infinity and at a closer measured distance, adjust all the screws the same amount so that the screen remains flat.
This is a not a technique that I would recommend to all because it cannot be accurate compared to a Hasselblad set up using factory tools but it may help you to see what improvements are possible so that you can see that having it set up to factory spec is worth while.
DON'T force the small screws, you really don't want to break them, trust me!