Depends on the hill. Well and the weight of you with the bike and what sprocket you have on the back wheel in conjunction with the wheel/tire diameter.
A good 50cc 2 stroke that has been optimized can scoot along pretty good and slight hills they can tackle without slowing down too much. Start getting steeper and the motor may need some pedal assistance.
Honestly if your goal is to have a properly legal 49/50cc motorized bike I would heavily consider the 4 stroke route. They both make about the same power and the 4 strokes are just a lot less to mess with and they get much better economy. This is coming from somebody that loves a good 2 stroke too. I say this because most of these 2 stroke kits out of the box require some tweaking and tuning to really get the most out of them.