RE: trying to decied on engine for tower trainer 40
the OS 40 and 46Fx are not offered by tower any more your LHS might have one for sale.
TT pro 46 which seems like alot of guys like.
the magnum which is a OS clone and takes a bit of break in time to really get running solidly there also suposed to be good runners once the break in process is done.
the 46AX
my choice would be the 46 AX if you don't mind spending the 104.00 worth every penny IMO, easy break in and loads of power.