This is what his base and super form for Kazuma Yuki's Maga Orochi and Maga Tano Orochi form will look like, Kamen_Rider_001.

In case you couldn't read the wordings, I'll spell it out right here to help you out.

[One spike fin for his base form, Two (spikw fins) for super form]

[Same design for other shoulder but tentacles formation will be different]

[Same design for other arm]

And yes, he will have eye visors extensions as shown on his super form, and his leg armour will be based on Burakawanii Combo, to add the touch that I found that Maga Tano Orochi has another head hidden at its lower torso.

They're rough sketches but still, this is what their forms look like within the Book Covers. It's base form is based on the image I found on Pinterest but I improved it a bit.

Welp, there ya go. You have my permission to use this for your OC, Kamen_Rider_001.