The best way I can suggest it to lengthen the MX83 wiring as you have suggested. Ignore the engine wiring.
This way the dash alternator charge light, etc will be all wired up already.
That said I personally wouldn't run the wiring across the top of the engine... if you do then if you ever want to take the engine out you will have to pull that wiring back out...(might take a little bit of effort once you tape it in and add split conduit over the top of it...).
I'd run the wiring around the top of the radiator support panel like I'm pretty sure they did on cars that came with alternators on the drivers side (i.e. MA61, MX62, etc).
Cheers
Wilbo
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