The board may be modified to allow operation 25 MHz operation by removing and adding several components. A skilled operator can do this in about an hour or less if the CPU is not changed. The proper approach is to use a $10,000 hot air SMT rework station for soldering or desoldering QFP (quad flat pack) devices like the CPU/FPU. The discrete parts may be hand soldered or de-soldered by an experienced person with an appropriate solder iron, but be aware that it is more difficult than working on "through-hole" components.
This modification has been successfully done on several PB 140's by myself and my friends. This was originally "figured out" by comparing factory boards from a 140 and a 170. These instructions are intended for those with some experience working on circuit boards.
The PowerBook microprocessor is U1, an MC68030FE16B in a 132 pin PQFP (plastic quad flat pack) package. Many of these 16 MHz parts will run fine at 25. Some may not. Motorola fabricates these parts and speed sorts them after testing. The devices are manufactured identically, some simply perform better than others. Typically the "yield" may be faster the part's marking, and the ability to pass all the tests across temperature ranges up to 65!C is the discriminating factor. If you do have to exchange the CPU (for an MC68030FE25B), desoldering the existing part is probably the hardest part of the job. It would be extremely difficult to remove the CPU or to install a new FPU without using an SMT repair station.
The PB170 is equipped with a separate FPU chip, a MC68882FN20A at U16. The PB140 is not so equipped, nor is the PB145, whose CPU operates at the faster 25 MHz clock rate. If the PB140 is modified to operate at 25 MHz, the current system software will recognize the modified machine as a PowerBook 145. Obtaining the CPU and FPU may be difficult, with the cost of both these in the >$50 range. For the discrete parts, if you can find an electronics distributor who will sell them to you, the cost is probably in the $10 range total. If you have a friend in the hardware business, these parts might be stocked in assortments of loose piece parts sold as "engineering kits". Remember that these discrete parts are normally sold on tape and reel (5,000 pcs) for SMT assembly machines.
These parts you _won't_ find at Radio Shack, so check with someone who knows good sources for electronic components. Mail order operations may be able to supply these, or certain retail stores such as Fry's in the San Francisco area. Most distributors will provide free samples to electronics firms who normally buy such parts wholesale. You might try getting friendly with some salesman, but the problem will be that the normal distributor does not want to deal with a $10 over the counter sale.
1 - 50 MHz oscillator (clock) module, these package styles vary a lot. The board can accept two different SMT footprints, or a 4 pin through-hole (0.3" x 0.3") device. The CMOS level, 0.01% part should work. Dale p/n X0-51BC50 is a part which will fit, a 4 pin through-hole with 0.300" center (square pattern) pins. The through-hole part is a lot easier to work with, and there are less size variations to worry about. Note that the memory board may mount over this device, check to see that you have clearance for any memory boards (these also vary in design) with the oscillator you use.
1 - 100 K= 15% (black, usually marked 104) 1/10 W resistor in a "0805" chip (0.080" x 0.050") SMT package. KOA p/n RM73B2A104J will fit.
1 - 1.0 uH inductor, 1812 (0.180" x 0.120") SMT chip package - Dale IMC-1812 1.0 uH 110%, or TDK NL1812-1ROM are two part numbers which fit.
1 - 0.01 uF (brown, usually marked A4, J4 or 103) cap (0805 chip). Murata Erie p/n GRM40X7R103K050 or Kemet p/n C0805C103J5RAC-3972 will fit. This exact part may be difficult to find, but any similar part may work. Voltage, tolerance, and marking are not critical, but every digit in the part number has a meaning. Most 0805 caps do not go this high in value.
1 - 47 = (black, usually marked 470) 1/10 W resistor (0805 package). KOA RM73B2A470J fits.
After modification, your 140 will "think" it is a 170 (or a 145 w/o the FPU), and you will operate much faster (6.07 vs 4.05 Speedometer CPU rating). Of course warranty would be voided (probably not an issue for those with the older model) and the modified motherboard would probably be untouchable by Apple technicians who have been known to look with great disdain at customer mods. Dealers don't normally do board level repair on these products.
So pursue at your own risk if you choose. Good luck!