An informed person might view this moment to have that chance to dial in the steering feel that they wish for. To do this well it will take some efforts. A fork of around the correct axle to crown (axle to brake hole plus brake hole to crown seat) length to nearly attain the top tube's slant, or horizontalness. Then minor corrections for final A-C dimension and trail (or rake).

I suspect that the top tube was designed to be fairly level. This then establishes the head tube angle, not that knowing it really is important for this. Dropping a plumb line from the axle to the ground and measuring forward to the front tire contact point gets you the trail. By measuring the mule fork, rake and a-c, one can then figure out what trail changes one gets with rake changes. Additionally the a-c dimension can be played with if needed. But this dimension's changes result in fairly minor steering angle changes. As long as the wheels intended are what the rear triangle ad brake reach likes and one matches these in the front handling geo can be played with to some degree.

But if this is all jibberish or "greek" to you then just get a fork that fits the head tube and ride the darn bike. Andy (who writes from 3000 miles from his desk).