I'm a signwriter of 34 years, I've painted a lot of bike logo's onto seats. Your local signwriter should be able to computer cut a stencil out of self adhesive vinyl.
It won't work very well with an embossed seat, but it does on the smooth seats. first clean the seat with solvent. Apply the stencil carefully & remove the application tape.
With a heat gun or hair dryer gently heat the stencil and with a rag carefully rub it so it moulds into the grain of the seat. Mask of the rest of the seat.
I use a spray gun, but you can buy aerosol cans of vinyl paint from your auto supply store. Spray very light coats, letting it dry between coats, building it up slowly.
It looks more authentic if you don't get full coverage. About 3 or 4 light coats should do it. When its just dry, carefully remove the mask. Should be perfect.