After struggle for some time with a resistor and a diode, trying to make the /WAIT signal work properly I have decided to go back and use the Flip Flop like in the original design.

The first flip flop (A) acts like an inverter and it is necessary to turn the negative going /ULARD  (/KBD) signal into a positive going edge to trigger the second flip flop (B) and generate the /WAIT to hold the Z80.

The first flip flop is said free because we only need one flip flop to latch the /WAIT signal and the 74xx74 has two gates.

The /ULARD signal also triggers the interrupt on the AVR (also on the falling edge).