First, get in touch with Buchanan spokes. They have all the specs and can sell you spokes to make the hub and rim work together. That is assuming the 19” rim is generally compatible in terms of the spike angles etc. 18 to 19 isn’t that big a deal, but the 19” rim needs to be drilled for a drum brake hub.
The different hub diameter and width changes the spoke angles, which means the holes in the rim need to be different. The small 350 drum hub is probably not much larger than a typical disk brake hub so chances are it will work.
Why the move to 19? When I built my 650cc XS cafe, I went with 18” rims on both ends. There is no advantage in going to a 19” front. Just heavier which means slower.