Headstock Bearings: Stick with replacing like-for-like. Apart from being much simpler to do, fitting races may need the casting boring out so much it will be seriously weakened. Also of course, if you sell the machine on in future, it may be more valuable for being original.
If you have the mandrel re-ground you'll need to make up new bearings to suit. Are the bearings adjustable? If so you may able to tighten them enough temporarily to make the new liners, having first measured the new diameter & made a Go/No-go gauge.
A better alternative would be to make the new bearings on another, good lathe of course!
Change-wheels: I asked Myford this very question as I have a 1908-pattern Drummond flat-bed lathe in need of TLC. Drummond & Myford wheels will not mesh properly. They may have the same DP & bore but apart from different keying, the pressure-angle is different. Try to find the original wheels' specs and match that; or make up a complete set separately.
Warning: Drummond change-wheel pins are in tapered holes and driving the pins the wrong way can break the wheel.