51mm *hinged* clip-ons, here's the Amazon link. You can only use 2 piece or hinged clip ons, else they won't fit because of a flange at the top of the fork tubes--plus you won't need to pull the top off the triple tree. I peeled off the METACYCLE black-on-black stickers on the fork tubes first.
I have one of these that I used to haul my old Honda CB300f around no problem with just standard ratchet straps. That bike weighed ~350lb. It was a little tricky to load by myself, but doable. I think the metacycle going to be even easier.
The lack of local pickup does kinda suck, but delivery and destination charges are pretty much industry standard. You buy a motorcycle or car from any dealer, there’s going to be a delivery fee that’s part of the price even though you’re buying it in person.
Can you measure the internal diameter of the bar? It's possible it's just a different common size.
(If you don't have one a set of calipers is very cheap. I've been using a similar set from harbor freight off and on for more than a decade)
You would be surprised at just how strong the adhesive backing on some of these mounting plates can be. And they really don't need to provide a lot of tension on the cable. Just enough to pull the cable away from the tire. The only issue would be if the inside of the swing arm were a rough or dirty surface and then the adhesive might not hold so well.
with slight mods maybe some hood latches would work??
Getting a tail tidy off of Amazon that comes with turn signals. Should be some easy wiring. Also taking off the fender/license plate holder. Here's the link
I think these two things ought to do it.
Abus 37RK/80mm Granit Plus Padlock https://www.amazon.com/dp/B006J3FW6O?ref=ppx_pop_mob_ap_share
ABUS Hardened Steel 12KS 6 Foot x... https://www.amazon.com/dp/B005UMCC7C?ref=ppx_pop_mob_ap_share