Sure you can. It's often legally protected activity. You're just limited to distributing your modifications without the original work.

For some games maybe, but software often has a clause forbidding reverse engineering

ChatGPT says that such clauses are typically void in the EU, though they may apply in some cases in the US. Even in the US, the triennial DMCA rule-making has granted broader exemptions for good-faith security research every cycle since 2016.

https://chatgpt.com/share/6838c070-705c-8005-9a88-83c9a5550a...