Can I use a 29er rigid front fork and still use 650b wheels (brakes are disc)? Any negative side effects?
With disk brakes you can easily run 650B in a 29er fork.
The main consideration is maintaining the bikes geometry. You should look for a 29 fork with the offset and a crown to axle measurement as close as possible to the original.
As you are changing to no suspension travel, remove the sag from the original forks measurements (typically 20%, 25mm for a 120).   From specs the fork you have has a 504mm axle-crown, less 25mm sag, you want a 29er rigid with about 480mm axle crown. (You do need to confirm these numbers by measuring your fork.)