There are some nice ICs that would replace the 555 and BF487 and some even the MOSFET all in one package. This will include current limit.
Do you have a oscilloscope? Can you see the duty cycle on the gate of the MOSFET? If you only have a meter you could measure the DC voltage on the gate. If the supply is 12V and you measure 6V then you have 50% duty cycle. My guess is you will measure about 1 volt. If I could see the waveform on the MOSFET-D I would know more.
Things to check. Phase of transformer. Primary and secondary may be reversed. What transformer are you using?
Is anything getting hot?