This is the exact engine I was hoping you would not have only for the sake of saying its the one I know the least about. Up here in Canada that engine option was not common (not sure if it was even available) due to emissions. here are the things I would be checking. if this unit has the e-controls fuel system, I would pull the carb diaphragm and wipe it with a clean dry rag. I know you mentioned the carb has been changed so this may not be a problem but this has been super common problem in the last few years as fuel gets worse. also I would change the spark plugs as I've had it happen to me more than once that even a complete new set of plugs died all at once. have you checked the fuel pressure at the regulator? One option to see if your spark and timing is ok is to give it just the smallest hint of ether while cranking and see if it kicks over. Hope this can be a bit of help to you.