The belt being too tight makes a HUGE difference to the speed of the motor. The little motors in these machines aren't powerful enough to fight a tight belt. vanginney is right, loosen the bolt and let the motor drop. A tiny bit of tension on the motor as you tighten the bolt is enough. This is described in the manual too:
http://ismacs.net/free-sewing-machin...ls-list.html#S
As an aside, even a full sized machine like the 66 I was working on last night will slow down if the belt is too tight. DH said "Give it the beans!" and I muttered, "I am!!!" so he went and got me the screwdriver because we both realised the belt was too tight even though there was lots of "flex" in the new belt.
That's the other thing... the old belt would be "floppier" than the new... it will need to be a little looser than where the "bolt mark" was for the old belt because the new belt will give more tension naturally due to the inflexibility of it. I think it will ease with time, but none of my belts are old enough to say for sure.