Hi,
Would you be doing the work yourself or getting someone to do the work?
In the long run, if you want a lawn, I would do a combination of both options... kill off the grass, rotovate, add top soil & reseed. Now is the time to start killing the grass off, will need a number of weeks for that to die off and recover before re-seeding...
If it's a quick cheap fix, then roll the garden to flatten out as much as possible, fill the holes with soil & sand. Have a look then to see if it's ok form your perspective - will need to seed the bare areas, but it's too wet, cold for reseeding at the moment...
I wouldn't try to cover all the garden in top soil at that stage, but does depends on the damage the cows did. As mentioned, just fill the holes to see how it looks.
Have you priced the top soil? For .66 acres - won't be cheap - as Commercial stated, you should be getting good quality topsoil.