Michflyer,
What you're looking for doesn't;t actually exist, mainly due to the aforementioned change up between standard and restricted categories. You could get an Ag Cat or Stearman in restricted category, and they'll do basic level acro just fine, but it's not legal, but that may not matter the way you're proposing to use them. Id say a restricted (agricultural) category airplane is mandatory if you're going to write it off. Its your one defense if an auditor asks how can your plane be a legitimate spray plane? There quite a few C-185s, and Super Cubs in the U.S that are dual category, restricted or standard based on whether or not you have the spray equipment installed. Although none are particularly aerobatic.
As for the Stearman powerplant debate, a 450 Stearman won't do any maneuvers a stock ship can't, it's just a matter of the skill of the meat servo behind the stick, and the fact that you won't lose as much altitude doing them in a 450. I fly crop dusters (turbine, not Stearmans), own a stock Stearman, and fly an ag-stearman with a 600hp R-1340, I much prefer the well balanced control harmony of a stock plane any day. the 600 feels like the control locks are still engaged.
Mike-