"You should exercise more."
i love it when there's more kanji for me to read, even if none of it makes sense, i can at least learn the meaning of the sentence.
duo is trying to make it easy still on some questions or they just haven't added the kanji yet, but ほうがいい makes NO sense to me and i don't know how to look it up.
if it said 〇した方がいい to start with, i can assume the rest searching 方【direction / way / side / area】
is there any kind of a service that can convert Japanese into full ancient kanji?
like こんにちは -> 今日(wa)
Nope. This is a perfectly normal construction and yours is wrong. You can do 運動する方がいい ("better if you do exercise"), 運動した方がいい ("better if you exercised"; this is also more casual, using た form as plain rather than past) or to negate it with, 運動しない方がいい ("better if you didn't exercise"). You only use の before 方 when you're comparing nouns, not verbs.