Knocking Briggs 15.5 Vertical Engine

Engine is a 31R707-0001-G1.   Had to replace the compression release cog.  Put everything back together, set the valves.   Getting a low throttle knocking sound which was probably there prior.  It isn't at the rocker arms.  No bent rods.    Seems like there is some play at the piston arm (lower) and the counter balancer arms seems like they have a little play as well.   How bad of a job is it to replace the piston arm?  And for the time and parts cost is it more effective to just replace the engine?

Appreciate the reply.   When you move the rod side to side it has quite a bit of play.   I'm not 100 sure the counterbalance arms are sound.  I can do it but when I add up the parts cost, my labor and just the time it would take to get everything off and then back on - I've seen new replacement engines that are in the low sixes.  I'm a little concerned that when I put in new oil, ran it for a few minutes and looked at the oil - it is VERY dark grey and looks like aluminum may be in the oil.   It immediately shouldn't be this dirty.   I used to use a service and bought this mower for 300.   It's paid for itself many times over.  My service charged 1950 per year.  If I get another three years out of the machine with a new engine it makes sense.    If I had the time this winter maybe the rebuild would be an option.  I just don't want to go through the part expense and time, put everything together and still have a problem especially if the piston and cylinder aren't perfect.   The last owner I don't think did a good job maintaining this mower.  He had a perpetual leak at the valve cover and maybe the main seal.   More than likely the engine's oil at some point was low and he still ran it.