The 46 and 42 are compleatly different blocks. The 42 is a much heavier and thicker casting than the 46. The 46 is made very light. Not alot of thickness to any of the casting. Just the cylinder alone on the 42 is 3oz heavier than the 46. Plus the crank and bottom cover are heavier. Here is a pic of the 2 side by side. The 46 is on the left.

I forgot to say, if his engine is a 2.1 and not a 42, then it is even heavier. The bore is smaller so there is more aluminum in the Cyl. I have a Poulan 2.1 here that is the same block as the 42. It is 4 oz heavier than the 46.