I have the same issue, and have read a couple posts elsewhere that an ST1100 side stand would work: but have not seen any details. So after finding a sidestand/spring on ebay a couple months ago, I tried it yesterday.
Short answer: It might.
Long answer:
Yes, it is longer. Original pic:

ST1100 version on same mount:

About 1.25" longer (there is some parallelax in the pics), and it does indeed bolt up. Even the safety switch from the Valk will attach:

However, the Valk version has a bit of an offset that the ST1100 does not; this makes the sidestand spring impact the Valk exhaust pipe cover:

I'm still going to see if this will work - have to get the ST1100 side stand to my welder next week. While I have a MIG I can only (half-assed) weld, while this needs an oxy-acyl rig to heat up the steel and make the same angle. May also have to find another spring, or maybe weld on another bracket to use the Valk spring. I'll post the results next week, I hope.