The 20th century is only marginally over 10 years ago. The average house that comes on the market described as "in need of modernization" is already 20th century, and all it would need is a thorough cleaning in order to make it look authentic. . In order to actually decorate, and still leave it looking 20th century: paint most of the paintwork in a brilliant white gloss, any exiting Anaglypta embossed wallpaper that is still in reasonably good condition with white emulsion, and replace any damaged wallpaper with new paper. . Cover any laminate flooring with in bedrooms living rooms, hall-stairs & landing with wall to wall fitted carpets, and use vinyl floor coverings in the bathroom(s) and kitchen. . Outdoor paintwork should be renewed with predominantly white gloss paint, and black gloss contrast on soffits etc.. . Best wishes - Majikthise. .
I guess it would depend on which part of the 20th century you want the house to resemble. For instance, a house in 1912 would look vastly different than a house in 1967. It would probably be very easy to decorate a house in the late 20th century. Vintage is pretty big, but I guess the real issue would be how much money you have to work with like Samantha said.
Wouldn't be hard if you have the money to do it. If you don't, then it may be a very, very slow process but you could always substitute cheap items instead of going broke on the expensive brands. Renovating can be more expensive and time consuming, especially if it is DYI project and place enormous strain on a relationship.
it would be hard if you don't have a lot of money