Figured I ask this same question on here too.
I only known about 3 or so fics so far where Spike got to be a Kamen Rider.
One which was a displaced fic with Decade and two others(that are sadly left hanging for years, in fact those 2 has Spike as the main protagonist) named Masked Rider Kind Of Friendship and Kamen Rider Drake.

Really wished there was more fics out there where Spike gets to be a Kamen Rider(and hopefully isn't ultimately in the shadows of others or playing second fiddle pretty much), it just hurts that Spike kept being left out a alot in many of the fics I saw.
Especially ones where its due to he's stuck as a freaking dog in EQG. Like others get to be Kamen Riders but Spike can't due to being a damn dog there..

So if anyone found any more fics where Spike gets to be a Kamen Rider, let me know please :0