Why do you think it *needs* a trimpot and how do you plan to adjust it if you fit one?
The addition of R13, D3 to the standard Vbe multiplier circuit adds a bias stabilisation feedback loop, so the bias should be automatic unless any part of the output stage and its bias circuit is defective or seriously out of tolerance.
The circuit currently has a trimpot in place of R11 & R12, so I was just planning to replace it like for like while overhauling the amplifier. However, when getting advice from various sources I have been told that a trimpot for bias adjustment is fine but that I should also reinstate one or the other, or both of the substituted resistors.
I have a multimeter, a function generator and an oscilioscope so I was planning on testing the amplifier thoroughly before connecting it up to my hifi system and as I understand it, the DC offset of the output can be adjust, if needed using the trimpot.
One thing to note is that while the amplifier is roughly true to the original schematic, it is not exactly the same with some components being of higher values, so I would assume that the original builder of the amp put the trimpot there for a reason?