Best idea is to look for another old mower that may be broke beyond repair and buy it for parts..... Last i checked a new starter ran a little over a $100 which is kinda high..... I have an 8 horse with electric start I paid $50 for [actually traded a cord of wood for 3 motors] so i got a bargain.... although ivwe never installed the motor on anything as of yet..... after 5 years of sitting there on project row.
Ive seen whole riding mowers in take running around here for $100.00 and up.... lots of parts on one for trading to other folks if you want to start a sideline business too.
and I guess you shouldnt rule out checking the online auctions, and even use those to mve spare parts.
William