I have been told that there is a well-known, simple mod for converting
an old Lafayette SWR & Field Strength Meter (#99-25835) and similar
meters into 2m/70cm SWR/power meters.

Ken-

I don't think a modification is required per se, other than removing the field
strength antenna. The instruments I'm familiar with are reflectometers,
similar to the design of many wattmeters such as the Bird 43.

You need to provide a power scale for the knob used to set full scale for SWR
readings, to set the full-scale power range. It may be as simple as using a
magic marker to indicate the settings for various full-scale readings such as
10, 100, 1000 watts.

If it has a field strength scale in addition to the SWR scale, that may be used
to read actual power. However, it will probably NOT be linear. In other words
it most likely will read voltage rather than power, so you would need to have a
conversion chart to convert from the scale reading to actual power. I would
expect to find that center scale would indicate one quarter of full-scale
power, but you will need to verify actual calibration.

For very low power levels, maybe one watt or so, you can expect additional
non-linearities due to the forward voltage drop of the detector diodes.

73, Fred, K4DII