What is the year, make, and model? What size engine does it have? How much do you want to spend?
Usually, adding new pistons with new cylinder gaskets and head O rings will bring power up to almost normal. But if you have bad crank seals, then the whole engine will need to come out (depending on make, model) and that will be a little more involved and take a few specialty tools.
Local sled shops can help too. Dealers are usually overly busy, so the nice little aftermarket shops are good, as long as you purchase your parts through them.
If you want to do it all yourself and order parts online, I would suggest finding the service manual and going through the step by steps. If you get stuck, come back here and we can probably help you through the issue.
And like BC_Dan said, it helps us to understand and help you if we know the make-model-year and engine type.