The first thing you have to figure out is how much 120v load there is. The heating elements are going to be the main load and they will do just fine on your 230v. That is the red/black. Once you determine the 120v load that is left you can figure out what size auto transformer to connect to the white. It may just be the light and the clock. If you rewired the light to 230v by moving the white lead to the red or black (the one you have on your grounded phase) you may be left with the clock as the only 120v load and it won't work on 50hz anyway so you could just cut that wire and not have anything on the white.
