Would a prophet head tube accept a 1.5 inch steerer diameter?

Yes, you’ll need to get a matching 1.5 headset too though. Lots if bargain 1.5 or tapered steerer forks to choose from, got some 1.5 Rev’s on mine

Oh & it’ll need an older “onepointfive” type headset, not the newer 44EEOS type.
Worth costing this & a new stem up, when considering this ‘upgrade’, as neither will be particularly cheap.
Cheap forks, but everything else ain’t!

It will also accept tapered forks, which I would reccommend for future frame compatibility.