Hi man. I had a similar situation - also went in blind! - when i got my XT600E engine to put in my '86 SRX4. The SRX4 is essentially the same engine as early XTs.
The good news is it is possible! However, at least in my case, there where a couple of hurdles to cross.
Conversion of two phase electrics to 3 phase - not that hard.
kill switch functions differently - old XT has a negative signal to CDI to cut the spark, while 'new' XT kills via cutting the positive feed to the CDI.
Your gonna need to mount the starter relay and wire it in, and while your at it avoid the 'safety' switches (Clutch, sidestand, neutral)
The pickup coil is incorporated into the magneto in early models not so in later models but easily rerouted.
Also CDI is very different.
Probably others Iv missed...
An autoelectrician did mine without any bike specific knowledge so with patience and relevant wiring diagrams definately possible, although you may have redundant wiring left behind...
Im currently trying to re-incorporate my kill swich, problem is will be reversed, ie kill will allow the engine to run while run will kill the engine if you see what i mean