Well, before you can start wiring in that reg/rec you need to know if the XJ650 maxum uses brushes. If it's the same as my 82 XJ650 Seca Then it uses brushes. It is the same reg/rec that is used on the 80 up XS650. Wire it up the same as you would the 80 up reg/rec.
On your reg/rec the three white wires go to the three white wires of the stator. The red wire to battery positive, Black to Battery negitive or a good frame ground. The green wire to the green wire to the outside brush.
Now you need to replace the three screws that hold the inner brush in with nylon screws. Now you trace the black wire from that brush back up to the large plug that hooks the stator to the bikes harness cut the black wire an inch or two back from the plug on the harness side. Hook a brown wire to the black wire from the plug. Run this wire over to the brown wire on the reg/rec. connect the two brown wires to a third brown wire, now connect this third brown wire to power after the switch.
Now when you turn the bike on power gets fed from the battery to both the reg/rec and the inside brush on the stator. From that brush through the rotor to the other brush, out the green wire to the reg/rec. Now as the reg/rec reads the battery voltage on the brown wire it will turn the power flow through the rotor by grounding the green wire in the reg/rec on and off to keep the stator output at 14.5 volts.
This keeps the battery charged to run the bike.
The yellow wire from the stator goes to the safety relay. On your 74 the safety relay controls the power to the starter relay so the starter cuts out as the engine starts and keeps it from working as long as the engine is running. This prevents excess wear and tear on the gears and shaft that hook the starter motor to the engine.
Leo