If you measure the diameter of the spline and count the number of teeth then you may be able to find an alternative that fits.
For an RSVR1000 I found
Yamaha 06-16 R6/02-03 R1 shift arm that fits straight onto the Aprilia gearbox shaft (part no 5PW-18112-00)
RS125 lever did look good as did the Cagiva Mito lever
MT03 (320cc version) shift arm
I will check diameter and no of teeth later.
Data Aprilia RSVR gear change shaft Spline 11.3mm OD 30 teeth