Oh he’s definitely going to pull a Donald Trump and contest the election on baseless claims if he loses.

Probably : "He’s stated that only God could remove him from office and has for most of his four years in government sought to undermine institutions that impose checks and balances on his powers. He has repeatedly cast doubt about the reliability of the country’s electronic voting system, even claiming without proof that the 2018 election was rigged against him because he didn’t win in the first round." - Washington Post.

He’s more on the “we got cheated” side like Trump. The current supreme courts were largely appointed by Lula’s party in the past and have taken some actually questionable action against Bolsonaro propaganda, which gave him ammunition to go back to a discourse of questioning the election. To be fair, before the courts acted to reduce his screen time and censor opposing media, he was turning the tables to win the election, so court action was pretty obviously politically motivated (and might have been worth it for the PT).

Rhetoric aside this is the only thing that is ACTUALLY at stake in this election, SC justice appointments: if Lula wins he’ll have full control of the top-level legal system again, if Bolsonaro wins he’ll have some control. Either will try to eliminate the other using this.