AYP yard pro deck upgrades..?

I have a AYP y2050b riding tractor 50in cut 20hp kohler and i recently tryed to buy blades...fail...everyone has to order them due to the 3/8? drive bolt hole and width of the blade...i was wondering if there is a way to upgrade the decks spindle and bearing to a new spindle that has the star key...and if that is possable what blades do i use to get a 50in cut with my deck and no lines between blades...

Thanks
Jolly!~!

part number for jackshaf 136819 i want to upgrade/replace