Yeah it can be a lot of things.
You say it sounds like the mower deck bogs down. Does this mean that the blade speed audibly slows down, but the engine speed does not? If that's the case you're going to look at both belt tensioners. Start with the primary belt....adjust the tensioner's tension, it is cable operated and the cable is adjustable. Also if the belt's worn, it'll ride deeper into the groove of the sheaves and sometimes will affect it's tension. Then move to the secondary belt. The secondary belt's tensioner rides on the same shaft that the double vee pulley rides on. Remove the tension spring and see if the tensioner arm is free. I hope it is. If it ain't, you're going to have to remove the double pulley and then use fire (propane torch) to free up the tensioner pivot. IIRC it's got bushings in it, which will melt when you heat it. Fine. Replace them with new ones, but before sliding the tensioner back down onto the shaft, grab a piece of sandpaper and polish the shaft up, then grease everything.
That's a start....
