I wouldn't change out the block unless you know there is a loss of compression.
Find someone with (or buy) a compression gauge and do a compression check. If everything checks out then you know the lack of power is elsewhere. If there is a loss of compression on any of the cylinders then you could be looking at worn rings or something on the head side. Squirting oil down the cylinder and performing the test again will give you a better idea of which. If the compression raises with the oil then the rings are most likely at fault and its time to look to swap it out.
The reason I say to do this first is because its very possible that your lack of power is due to a problem elsewhere.