Making more players in game module

[BrudasekPL]

Hello. I've got demo of awesome game that's 1P only, and I want to play it with my bro. In options there's label PLAYER NUMBER:1 BY MOD. Can I make option that will make 2P game possible? Or shall I contact demo creator?

 

[SaintJudas]

What's the best version of the engine to play this with? Anyone know?