To do the job right you need to know the magnetic characteristics of the core material. The flux density you want to use, the effective cross sectional area of the core. Armed with those numbers you can calculate the number of turns, and all the rest. Formulas are available in many texts.
The wire size will depend on the current you expect to flow. The power handling is limited by the window area and wire resistivity.
Proceed with the traditional calculations, verifying the load current before the secondary winding. At the very least increase the coils. I have an old amplifier with 2sj50 / 2sk135 but still have not decided on the configuration decisive.