If you are good at welding and fabrication I don't see any reason why you can't build a satisfactory rotor for about 8" diameter, I thought you were looking at something smaller.

If you have to make do with scrounged parts then it will be pure luck, but building  a basic runner should be reasonably possible. Possibly the radial flow version of Francis ( originally Thompson) may be easier than the partial mixed flow versions.

You would need a bit of ingenuity and a fair bit of facilities to make a true kaplan but a simple propeller rotor is not difficult and if you have to make a few attempts at blade angle to suit your conditions it wouldn't cost the earth.

Attempting any form of volute chamber would tax your skill to the limit, but you could try an open topped concrete scroll with steel guide vanes and if you can keep enough water in there to prevent a vortex drawing in air it should work ok, just use your pipe as a draught tube, keep it under water at the tail race and a conical expansion section would help but may not be worth the trouble.

If you can have a versatile means of matching your alternator to variable speeds it will help a lot to match the turbine runner.

Give it a try.