New to model railway after a lengthy gap so a steep leaning curve om occasions. Concur with Ultrascale fitted diesel conversions although having waited for my Dapol class 52 wheelsets to arrive, I found my model has the later 1.7mm stub axles, not the original 2mm which the Ultrascale wheelsets are designed for. Luckily, original gear wheels are still available but I did manage to drill one the new style gear wheel back out to 2mm. On Ultrascale lead time, now 8 months..!
Once I have converted all my diesel stock, several Ultrascale orders in the pipeline, I will begin in earnest on the steam stock. My approach will be to leave the original motion gear in place and adapt Alan Gibson wheels to take the original RTR motion gear.
I did partially convert a Bachman 5MT quite quickly in order to test a British Finescale B7 prototype turnout. Alan Gibson wheels on tender/pony and original RTR drivers with adjusted back to back; AG drivers will be fitted at a later date. Not a brilliant runner but it tested the turnout..! Video on RMWeb and on EMGS virtual show demo; https://emgs.org/virtual-show-2021-welcome-closed/virtual-show-2021-demonstrations-point-kits/.
I am also part way through converting a spares/repair split chassis 4MT to EM and DCC. Tried the EMGS worksheet route but too many issues as a few parts have changed since the note was first published so I have a plan B.
The key point is you never know what you can do until you try..! Good luck.