The early /7 and /6 models had spoked 18" rear wheels that could fit on the rear, but would likely also require an axle and spacers swap. Additionally, you may also have to change out the rear brake shoes as I don't know if the rear brake drums are the same diameter.
The same principle could apply for the front, but it gets more complicated there as the spacing between the forks legs and the spacing for the front rotors is not the same as with the /6 and /7 forks, and those spoked wheel bikes had 19" front wheels instead of 18". One of our members here has run with the 19" wheels by dropping the fork tubes about 1 - 1.5cm in the fork braces to somewhat compensate for the added height. And the brake rotor offset for the swinging caliper ATE front brakes that were on the /6 and /7 models is not the same as for the Brembos on the later R65. So - you'd have to do a considerable amount of parts swapping and changing, along with the axles - or possibly have to have some custom axles made up. The 18" spoked front wheel of the 1981-1985 R80 ST is the closest to fitting directly, but may still require fettling with rotor carriers. The hub of that wheel itself is the proper width and setup for the same front axle, I believe. It can be accomplished, but switching back and forth between the two setups would not be a simple task - and realistically, it isn't that quick to switch between sidecar and solo configurations with most hacks either.
Actually, it has been discussed that the spoked wheels may be better at handling the side loads created when driving a hack than the rigid, cast wheels. I don't know this for certain, but I am preparing to fit a Sputnik sidecar up to my snowflake-wheeled R100/7. Everyone I've talked to, no matter their original intentions, ends up with keeping a dedicated bike with the sidecar attached, and getting another bike for solo use.
