The amplifier power will be split between whichever speakers you use. If you plugged it in to a 4x12 cab, the 120 watts would split 4 ways.
With 2 speakers, they will each get 60 watts. A bit of a safety margin helps, you might consider two 75watt speakers if you play loud.
But just as important is the speaker impedance. The amp is designed for a 4 ohm minimum load. So you would want two 8 ohm speakers wired in parallel to make a 4 ohm load. That is probably what the stock speakers are.
Two 4 ohm speakers wired parallel would make a 2 ohm load which would overheat and damage the amp. They could be wired in series and make an 8 ohm load, which would be safe for the amp, but the amp will put out a little less power into 8 ohms than it will into 4 ohms.
With 2 speakers, they will each get 60 watts. A bit of a safety margin helps, you might consider two 75watt speakers if you play loud.
But just as important is the speaker impedance. The amp is designed for a 4 ohm minimum load. So you would want two 8 ohm speakers wired in parallel to make a 4 ohm load. That is probably what the stock speakers are.
Two 4 ohm speakers wired parallel would make a 2 ohm load which would overheat and damage the amp. They could be wired in series and make an 8 ohm load, which would be safe for the amp, but the amp will put out a little less power into 8 ohms than it will into 4 ohms.