Depending on your budget, if you have the money to spend, using a rigger may avoid you some headaches and give you peace of mind that it's probably done right. You might want to get a few estimates to determine cost.
On the other hand, I don't think it's that difficult to do yourself. You can get the
line lengths from the original specs for Ericson boats (the Ericson owners website is very good and helpful), and a good chandlery or rigger can advise on options for what kind of
line to use for each one you are replacing. Replacing the sheaves is not too hard, though you will have to be hoisted up the mast unless the mast is down for some other reason.
Good luck!
Frank.