I think the lack of response shows that the answer is not straightforward.
The easy solution would be to replace the wheel.
Unfortunately Precision Paints, the owners of the Sharman tooling, have no plans to re-release these wheels.
So spares are a problem.
I had to dig out an ancient kit in progress (an awkward Jidenco Z, hence the 25-year procrastination!) to find any Sharmans in my stock.
I think they came with the crankpin pre-fitted.
So digging the old one out without damaging the wheel will be a major fiddly exercise.
To replace it you are likely to have to fit a new crankpin from the rear of the wheel.
And you need to think carefully about this.
If you fit an Alan Gibson crankpin, they are metric (M1) not imperial.
Some people say this makes no difference but I beg to differ.
If you go for Ultrascale be prepared to pay and possibly wait for months for supply.
I have no experience of Markits.
You might give yourself more problems than it is worth to have one crankpin that differs from the others on the loco.
It would be worthwhile trying to find a bolt with the same 14BA thread as the Sharman crankpin and fitting that.
I recommend using a taper tap (available from Eileens) to make the thread from the FRONT of the wheel.
Let the taper gently find the existing thread in the wheel and screw through to the back.
(But others may have a better solution).
You might need to carefully drill through the wheel with a narrow <0.75mm drill to guide the taper through.
(Do not damage the existing thread).
Then screw in the crankpin from the rear of the wheel.
You may need to make a countersink at the back of the wheel to take the new screw-head.
Can anyone confirm the specification for the thread on the Sharmans is 14BA?
(I tested mine with a 14BA nut and it seems fine.)
Why can I only see 7 wheels for that Z…………?