I'm not saying this won't work, but I think you've got to forget about the Koa being a retro bike and build it as a modern bike.
I don't know the precise a-c length of a 100mm Reba, but it must be c47.5 cm, say 44.5cm with a sensible amount of sag. The frame was designed for 41cm, so you'd be reducing the head angle from the design target of 71 to 67 static / 69 sagged, and you'd increase the bb height from 30cm to 32.5 static / 31.5 sagged.
With those kind of angles, you need to forget the flat bar/flat stem retro style, and go for a short stem and swept riser bars (maybe you have already?) This speeds the steering up again and is the best way to make a 67/69 head angle work. Although of course the riser bar adds to the height of the Reba and will make the riding position hugely higher at the front/weight further back. Nothing wrong with that, it's the modern style, but it would feel weird at first.
With lightweight steel bikes, there can be an issue about rear disc adapters putting a potentially fatal load on the seat stay, but I presume (I'm not pretending to know) that the Koa's plain-gauge 7005 stays are strong enough for that not to be too much of a worry.
I don't think it's ideal, and the best way to build a modern bike is to use a modern frame, but it could be a decent bike. Aside from the bb height, the Koa is very much the same shape as a modern Cinder Cone/Blast, it's just not built quite so strong that's all. Anyway the only way to find out is to try it I guess. If it doesn't work the way you like it, you could always put the Reba on another frame.
