If the previous owner of these units was the one who did the mods, it may be possible he/she still has schematics or photos available. I'd give them a ringy-dingy.
In the absence of a decent spectrum analyzer, an inexpensive garden-variety SDR that covers 300 kHz-30 MHz will do well to sniff out spurs. But if you can't undo the mods done by the previous owner, you may need to fabricate a steep low-pass filter to attenuate whatever harmonics you discover, and call it a day.
Be aware that patching a patch will mean it is no longer FCC type-accepted. Proceed with all due caution.
i now have hi res pics and schematics and parts are on order and will be here today. i'm good to go on arrival of parts.
Billy said: "Be aware that patching a patch will mean it is no longer FCC type-accepted. Proceed with all due caution."
I said: That's not entirely true. Any modification to the RF circuitry would require a "permissive change." Repair would not.
AMRL - True that restoration to the original circuitry would probably retain type acceptance. But installing a filter to eliminate spurs created by the mods (a "patch to a patch") drastically alters the design enough to put certification in jeopardy.
If The Man With The Payin' Paper shows up, we got some 'splainin' to do.