Any of the 81-84 final drives will fit. Final drives from the 79-80 R65 bikes will probably also work, but may require different spacers and axle. All the R65 bikes from 79-84 came with either 31/9 (3.44) or 32/9 (3.56) final drive ratios, which need to be used with either W1276 or W1318 (respectively) calibrated speedometers for the odometer & speedometer readings to match that of the final drive.
You could probably also put a similar year R80 final drive on (usually either 37/11 (3.36) or 32/10 (3.2) your R65 but your acceleration will suffer a bit (and you'll need a different calibrated speedometer, too).
