Rewiring 84 harley gas gauge

Try this, I do not know if they would have anything that far back but at least it will be a start in your search.
HD service manuals at http://www.downloadninjas.info/service-manuals/harley-davidson-sportster-xl-xlh-service-repair-manual-for-1986-2003-models/

Try this site, http://www.bikebandit.com It has exploded views of every section of the engine.

I don't think the '79 model had an oil filter. The '80 & '81 model had a bracket that had a filter between the rear cylinder and the oil tank. The 78 and earlier models had the drop in filter in the tank. I think you can put one on if you change the front engine brackets to '82 to early '84 models.

Good Luck
Steve

the fxr 4 gallon tank fits with 2 extension plates at front of tank ,this I used on my 92 sportster hugger