Harley-Davidson had Suicide Machine of Long Beach Ca build these from Street Rod 750s, and a snow track kit.
The bikes started off as bone stock XG750 Street Rods from Harley and track conversion kits Camso DTS-129 from a dealership in Salt Lake City, Utah
It's not a painfree bolt on though, because the the Camso units are built specifically for dirt bikes, not street bikes, which means the track unit does not have a straight chain alignment, but if you know a machinist, or have some access to some mills and lathes... then make spacers and other adapters to get the drive chain aligned with the bike's primary drive.
This meant that both the bike and the track unit itself would have to be modified to make the project work, start by removing the Street Rods' swingarm, rear tire, and shocks, then fabricate a strut with quick-release hardware to connect the track unit to the bike.
Up front, remove the front wheel and fender to mount the conversion kit's ski, using the stock Street Rod axle, forks, and triple trees and you'll need to make some custom machined spacers.