I just bought some adhesive seam-guide stickers from Sew Classic. I'm going to adhere them to my unmarked FW needle plates. There are 5 for $1.29. Thought it would be worth a shot, since I paid $40 for a vintage marked needle plate and have two more FWs that need markings! If you type "sew classic adhesive stickers" into Google, it should come up.
Ila
p.s. I have no connection to Sew Classic other than being a first-time customer.