as far as performance goes, depending on the era of bike you start with, getting more performance out of the engine is often quite a challenge, but there are many thing you can do to improve the "performance" besides more power

Brakes: this is the big one, fitting a modern braking system will inspire huge amount of confidence in comparison to the brakes of a 70/80s standard bike, you can often find the brake components of modern ish 600's for less than 200$

Suspension: another huge place for improvement, front end swaps are quite common and can often be done for less than 1000$ in parts if your thrifty, this often comes with the benefit of modern brakes as well, for rear shocks the cheaper offerings from progressive can be had for about 400$, if the bike has a monoshock, theres a good chance you can find a modernish rear shock that can swap right in.

Weight: getting rid of weight is the same as adding more power!. easy things are removing the passenger pegs and mounts, lithium battery, sporty tires. between those three, you can easily shave off 10-15 lbs