Hi, I see Jeff has not gotten back to you on this so in the mean time, allow me to see if I can help.
There are a few things that can cause the starter to not engage the engine.
First, is the solenoid kicking out when you hit the starter button?
Where reference #1 is the solenoid I am referring to. The bar in the center that has the hook in the end should jump out when you hit the starter button.
2nd, Is the starter clutch freewheeling in both directions? It should freewheel one direction and lock up the other. If it is freewheeling both ways, replace it.
3rd check the starter gear, idler gear 2 and compression spring. Check the gears to make sure the splines are not stripped on the inside of the idler and outside of the starter gear. Check the spring to make sure it isn't sacked out. (Items 5,7,9 in the same diagram as above)
4th, if you have had the flywheel off the end of the crank, make sure it is not slipping on the crank now as you try to start it.
You can put a socket on an extension on the end of the starter gear to keep it from spinning off and actuate the starter system with the cover off. If you are truly lost, it could show you what is wrong.
Let me know how that works out for you.