Overalls are a great idea and playdoh, putty, or different consistencies of slime are perfect places to start. Another thing you might want to try is a thick glycerin lotion on his hands. I think you live in Canada, my friend - sorry if I'm misremembering. Try something called Glysomed - it comes in a bright green tube and you can get it at Shoppers for sure (probably Walmart as well). I've also used hand cream from Avon called Silicone Glove, but I prefer the Glysomed for kids. Help him put some on his hands when he's on the potty but don't completely rub it in.
If you think part of his smearing is related to scent you can try basic room sprays, an essential oil diffuser with a scent he likes (my go-to method), etc. Some people try a chunk of strong cheese but to me that's just asking for trouble. I know some people have tried things like worn socks or shoes, but it can be tough to determine what type of odor he's seeking out (strong vs pleasant).
Some OTs recommend drawing, using stickers, or finger painting as being good for visual stimulation. There's nothing wrong with giving one/all a try, I just tend to go for methods that are going to be easier to bring with you when he's away from home and might have to go (a small tube of lotion or pot of slime as opposed to a pad of paper and crayons).
General suggestions: don't make him participate in cleaning up (could just prolong the positive sensory experience) and try to be neutral in your attitude. So don't offer negative or positive attention - just kind of acknowledge it, get him cleaned up and then out of the room doing something else while you finish with the bathroom.
Try to view potty training as a side quest (if you're a gamer) or a special challenge you need to unlock to move to the next leveI. And always, always try to view it with humor (I know it's so hard and frustrating sometimes!) I freaking love potty training - it's such a rush when a kiddo is able to accomplish it. It takes time and dedication, but you can do it!