1) Treat gam mak as purely funtime hobby, only work when the mood strikes you, and maybe never finish, but it's OK because it's just for funsies
b) Instead of waiting for motivation, get into the habit of making gam on the regular. Set aside a time that is specifically gam mak time, and spend that time on gam mak, no matter what. Habits are more reliable than motivation, so you'll be infinitely more likely to finish.
III) Join an event, work like a man possessed, make a gam that's subpar but finished thanks to the motivation of deadline panic.
All options have their good and bad points, and all can be appropriate for different projects.
In the end, if you're super serious about finishing a thing, you need something other than motivation.