Hello.
Take a look, it may be errors in there.
The 100k pot, will adjust the frequency of operation, from about 650 Hz to 23 kHz.
The 555 will output about 3.3V, like Ron H said before, it will not be enough to drive the power mosfets. But is enough to drive a BD139, so Q1 will act like a "voltage amplifier", and Q2 will act like a "current amplifier". (We are not interested in the linear region of the transistors, because we'll work with pulses.)
There is no problem to parallel mosfets (we do not need source resistors).
To obtain 12V and 5V from the 26V supply, just use the 7812 and 7805 ICs.
The main problem is that the duty cycle is not aways the same, it will vary from 45% to 67%. (Will vary as the potentiometer varies).
View attachment 8610