No Power

I have a Scag STHM (3 wheel rider), 61" deck,with a Kohler command cv22 basic engine. Over 2k hrs, 17 yrs old. Still mowed like a beast. I was mowing awhile back and lost power, limped it to the garage and eventually had a shop rebuild my engine. I pulled the cover and the spring and lifters were shot. The shop put on two new heads and guts, and returned it to me. Engine sounds great, definitely stronger but when I engage pto, I lose power. I have mowed and it runs fine but it will lose power, now I cannot even get it up to speed with the pto engaged, drop pto and engine roars back. I have not paid my bill yet because my mower just ain't right. I'm hesitant to ask them to look at it and definitely don't want them taking it because of some things they did that I was not happy with, plus their communication sucked, and they took forever to "fix" it. I had to fix a few things once returned. I had to put on a new battery, reroute my steering cables properly, straighten my steering arms, replace my battery cable because it laid on muffler and burnt through and started sparking when mowing, straighten out my forward/reverse linkage that he bent. I did have them come out once because it was off, they pulled the solenoid and it worked, some debris was in carb, and a fuse was blown. It ran fine after they left, then started bogging again, so I replaced all fuel lines since they were original. It seemed ok for a bit, then bogged. I changed the oil before 10 hrs of use for break in and it was nasty. Then it ran for a bit and bogged, and now we are up to date. I want to pull carb and check and replace both spark plugs since I'm not sure if they were replaced, they don't look new.



What else to check or do?