What is the biggest engine you could realistically drop in a classic mini?

i was thinking about dropping a new engine into my mini, but not sure how big i should realistically go, i have seen things like a 1.8 vtec engine. but could someone tell me what the best/biggest engine you could drop in without modifying too much.

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).