Whilst I haven’t converted a Q1, I have converted a number of Hornby locos and the principles are pretty much the same for all. Firstly check on the axle diameter. Most are 3mm but some, such as the J15, are 2mm and Alan Gibson generally provides wheels with the correct diameter centre hole. Then apply the basic rules Pete Hill sets out in his conversion sheets (I think some are on this website and some can be found on the Alan Gibson website) and you are unlikely to go far wrong. If using Gibson wheels, note they are not self quartering as the Markits wheels are, so you would be well advised to invest in a wheel quartering tool such as the one made by GW Models.
regards and good luck