First things first buddy, could you drop the caps?
To "restore" power, you first have to know how much is lost. I'd start by changing the plugs and filters, which shouldnt run more than $100 and should be easy enough to do yourself.
Next step, I'd look into rebuilding the carbs. Probably a couple huindred for a rebuild kit, if you are willing to do the work yourself.
Next step would be to check the valve spacing. You are looking at about $100-$200 for the shim kit, should you need adjustment, but the procedure is somewhat complicated. You'll need to remove the intake and exhaust to gain access to your ports.
From there I'd go to a compression test, and see if all your cylinders are sealing well. If there is a problem there, you'll need a complete top end rebuild, which means removing the engine from the frame, and pulling off the top end. You will either need to lap your vavles against the seats, or replace the piston rings, and if you made it that far, you might as well do both.
Bottom end rarely has problems that cause a loss of power, however it could have problems that are waiting to turn into catastrophic failures, so check for any tapping noises.
Hope that helps