A 21g servo in a small lightweight model? I don't think so.
There's 7 or 8 g servos available at very low cost but those might be on the heavy side for this.

Anyway, I think converting an existing airframe would be quite a project. Better start from scratch. Or get something that was designed for R/C in the first place. If you want small, I quite like the look of the Microaces range of micro foamies: pretty scale WWI models and a micro trainer. https://microaces.myshopify.com/coll...aces-aero-kits

Edit:
you might find some ideas in the Electric Indoor & Micro RC Flight and Park Flyers & Backyard Flyers forums here. I see there's a Guillows Chipmunk conversion thread in the Micro forum.