I have an old Nashbar mountain bike with a suspension fork. I would like to replace the fork with a rigid one. The suspension fork weighs tons and is too squishy since I ride it mostly on trail paths. Since the bike has disk brakes I am hoping that an inexpensive 29er fork will make up for the height added by the suspension fork, keeping the handling about the same. Any thoughts?
If you are keeping the same 26" wheel, the important dimensions are crown to axle height and rake. Aim of the exercise is to keep the head set close to the same height from the ground as it is now, without affecting trail.
Rake affects trail which affects handling, so with a different rake you could get a bike that handles differently, in the extreme, becoming unrideable.
When comparing the crown to axle height, you need to account for the suspension sag (normally 20-30% of travel). The difference in wheel diameter of a 29 over a 26 is 63mm, so a 29er fork will be around 30mm higher crown to axle (presuming built for same tire size and not suspension adjusted).
You will not want a suspension adjusted 29er fork.
If you are replacing the wheel with a 29er, you need to account for the increased ground to axle height. Also when changing wheel size, you increase trail, although its probably not enough to affect handling.