In the early 1990's there were statistical analyses programs sold that had historical lottery/powerball draws going back. The simpler theory was that knowing that certain numbers had come up with greater frequency than others either showed you the game was not truly random, or else the lower frequency numbers were somehow "due".
Before the consumer internet this sort of series data and similarly, stock prices and horse/dog racing, was manually maintained and sold at often high prices.
If your protagonists know exactly when they're travelling back, they only need to access the local newspapers for the period of travel. o.m.'s excellent answer suggests microfilm. Many major daily newspapers are imaged and available online as well just not yet text searchable.
If you can handle a 1990 travel date you could also consider accessing a FidoNet Echomail archive. Actually someone in 1990 is bound to have published the previous 10 years winning lotto numbers back then so you can have your 1980 travel date.
Usenet was founded in 1980 and rec.gambling was created in 1990 alt.gambling was founded one or two years earlier. Again there should be an archive somewhere of the data, so unless you want a quick and easy search for your protagonists, making an obscure find is one of their quests.
Another idea is to find a lottery that was superseded/rolled-up into a bigger lottery like MegaMillions. Search for results on the earlier lottery.
Lastly sports betting in the 1980's was huge in the Australia, UK, USA and Middle East. Especially Thoroughbred Racing. That data is easier to find. It was not unusual for Group 1 races to have individual million dollar wagers. A 1980 $100 note can be turned into several million in one day.