Advice Needed On New Replacement Filter

You can have an external, there are some deigned for small tanks. With 60 litres look at the filters rated for 300 to 400 litres per hour turnover. You can get bigger if there is a flow adjuster to turn the flow down. The thing to avoid is turning the tank into a washing machine with the fish plastered against the opposite wall 

When you replace your current filter it is important to remember that the majority of your bacteria are in the media inside the filter (sponge, whatever is in there). Take the media out of the old filter and put it in the new one. An external will have plenty room. Cut up a sponge if necessary, put everything into the new filter then fill up the spaces with new media that comes with the new filter. If you don't want to leave the old media in there permanently, wait around six weeks, then swap a bit of the old media for new; wait a month then swap some more and so on till all the old media has gone.
Keep the old media wet between removing the old filter and putting it in the new filter. Monitor your ammonia and nitrite levels for several days as moving the media can sometimes upset the bacteria a bit.