Seeing it's got a crank mounted clutch (the giveaway is clutch arm on the front of motor) it most likely won't be any bigger than 125cc. Probably better gearing (sprockets) makes it feel faster than your old 150cc. Check the engine number, it will probably start with something like IP56********, the 56 stands for a 56mm bore. Should take between 700-800 mls. I measure mine with the bike level (not leaning on the stand) and the dipstick put in but not screwed in. The oil should be in the middle of the cross hatched section