The usual method is to shim the seatpost out to a regular stem size, if you can find a nice 1" steerer AHead stem the shim will be thin, but there's no harm in going all the way out to 1⅛"
Shims! Of course. As if my go-to was to get something machined for me. Evidently spend too much time with engineers.
Seems easiest to go 1 1/8”, more choice.
The usual method is to shim the seatpost out to a regular stem size, if you can find a nice 1" steerer AHead stem the shim will be thin, but there's no harm in going all the way out to 1⅛"