For attics, there are three issues that need to be addressed: 1) vapor barrier, 2) air barrier, and 3) insulation. #1 and #2 can be handled together--typically with a poly barrier just behind your ceiling drywall, or with kraft-faced batts. I'm betting that in a 70-year-old house, you have neither.
If the blown-in insulation is the only insulation in your attic*, then this is what I'd suggest:
1) vacuum out the old insulation **
2) install soffit vent things that go between the rafters and keep the insulation away from the underside of the roof decking, so there's a path for the air to vent through the attic
3) put down a poly vapor barrier across the whole attic, and make sure it's sealed everywhere***
4) blow in a boatload of insulation, including the old stuff (you can reuse it)
Are you willing/able to DIY some of this work?
* our attic (only 10 years old) has kraft-faced batts, then several inches of blown-in fiberglass.
** you could also rake all the insulation to one side, put poly on the now-clean side, then rake all the insulation back onto the poly, exposing the other side, and put poly down on the 2nd side
*** an alternative: get a thin (2" or something like that) layer of spray foam, to ensure that all gaps and penetrations are sealed