there's supposed to be a blue part there with a tiny lens to converge the light into the photocell (I presume)
Yep, looks like it. And the light meter is unlikely to work well without it, so I wouldn't count on auto-exposure working properly on this camera.
You
may get somewhere by figuring out how much it over/underexposes by shooting some test frames and then adjusting the ISO. This
may get you in the ballpark for
certain lighting conditions, but since the lens also affects the angle of view of the photo sensor, the meter will likely be much more directional and hence thrown off easily by relatively bright or dark areas it happens to point towards. And you
may improve the situation a bit by sticking at thin, translucent diffusor over the sensor aperture, which should make it less directional, but also less sensitive.
So, basically, a lot of uncertainties.
For testing, you don't necessarily have to waste an entire roll of film. You can split a roll of 36 exposures into multiple smaller rolls if you have a changing bag or darkroom, and some spare 35mm cassettes. Just tape the leader of the roll to the spindle of a recycled cassette (ensure proper orientation) and in the changing bag, pull out the entire film from the original cassette. You now have two cassettes linked together with about 1.7m of film. You can easily find the middle by taking both cassettes in one hand and pulling the film in a loop with your free hand, pulling the loop tight and thus finding the center. Cut it there, and manually spool both ends of film into both cassettes. This will give you two approx. 15-16 exposure rolls. You can split these rolls once more in the same way, yielding two 6-7 exposure rolls.
If you use film you're familiar with and perhaps can also expose correctly in a known-good camera, you can compare the results and determine whether your camera over- or underexposes, and you can also make a ballpark estimate by how much, approximately. Adjust ISO setting on the Ricoh to correct for the deviation and try again, see if you can get any closer.
Alternatively, store this camera away until you find another one so you can maybe combine two cameras into one operational device. Or sell it off as a parts donor for a token amount of money.
Welcome to Photrio BTW!