Useful links:

PANADAPTER
Panadapter SDR per IC-751A from TSP S.r.l. www.tspelettronica.com

VARIOUS MODS
Modifications to the IC-751A for the Digital Modes – Squash Practice

TCXO mod
https://www.mods.dk/view.php?ArticleKey=1604bc2dc103f9365ef93cc0df15fcae

ERROR IN SERVICE MANUAL
https://www.youtube.com/watch?v=MfCW7DUaMeM

Mod for connecting an audio board and allow digital and MIC operations without disconnecting the Accessory connector (20k resistor added in series in AUDIO IN):
https://franck.aquarelles.org/?p=581

Schema for allowing the complete control using a single USB (except for the Winkeyer). The dashed boxes could be put inside the IC-751A (space permitting):