Possibly you are asking to much of your knee like this could be the early stages of healing.
Both my knees shot (left hip & left ankle as well) due to work & m/c comp incidents. Often found myself riding shortly after said operations. Sometimes used the more standing up position of an Observed Trials, but then my cafe racer to screaming 350 RD/RZ Yamahas, & whatever else.
I do not think there are special kits to lower the footrest, but you can look at it & think of what a few inches lower, by an extension in some metal should do the trick though You might have to make some changes to the bar to the cog box.
Think it over & think of a small time machinest & what he can possibly churn out for you. We did it many yrs ago when we were converting normal Brit street bikes to road racers what with our own designs of rear set. Thanks to a lathe & a bit of metal we churrned them out including the linkage & just so many other things we had to change in the bike for road racing form.