I would think about buying a new male. Avoid mom and pop stores, my luck with them has been bad. I would look for perhaps some local feeder breeders to buy a young male or two.

Quarantine as far as possible from your current colony. Don't even where the same clothes between the two. Treat the new males as a huge risk, cross contaminating nothing between the two. I would even go so far as to get a fecal done by a vet. Treat them for parasites, internal and external. Keep them in QT for a minimum of 3 months, I'd personally go longer.

I'm not 100% sure on the nature of most rat viruses, but I believe many are shed after a period of 3 months, and then are killed off by the immune system. Many other viruses are kept in a carrier status... so there is a risk to just throwing them in after a period of time.

When you feel like enough time has passed, introduce a few weaners from your present colony to the new male rats. Keep them away from your colony still. Watch for signs of illness, this may be a good way to test if they are sick at all.

Just a few ideas, I'm sure others will chime in too.