There is something about classic vehicles that for many is a strange contradiction. On the one hand, a 40 or 50 or 60 year old performance vehicle is clearly not competitive even with today's commuter appliances. On the other hand, many of us have a strong desire to increase the performance of a classic vehicle to the point where it is better than showroom stock. You have to ask "why" ? Why would you want your 1964 Verbrühten Hund Sport to be 68 HP rather than the stock 57 HP, reducing the 0-60 time to 13 seconds from 17 ? Good question, glad you asked.
The first answer is that you want to unleash the beast within. Although you love the vehicle, the bean counters at the factory clearly forced the engineers to install mild camshafts and puny carbs, and bad vinyl so that little old men could afford it on a measely pension. After all, you are no little old man. You are simply realizing the potential of the vehicle by applying the dollars that the bean counters withheld. From the time effort and money that goes into some classic vehicles, the bean counters must have withheld funds exceeding the entire cost of the vehicle.
The second answer is that you want to make it usable in today's driving/riding environment. To do this, you need to increase the horsepower by some significant percentage. You also need to install modern tires, you need to upgrade braking, and you need to upgrade the lighting system, which means the two mice in the wheel have to go, etc. You also need to replace the points with a pulse generator unit that operates based on anti-matter. It is a funny thing that I have never been left stranded by points (emery cloth solved some poor running on several occasions), even though I have replaced them in several vintage vehicles. The condenser is another matter, but I digress.
The third answer is that you want to have the most powerful 1964Verbruhten Hund Sport in the club/region/country/world. You want to be able to offer boasts like “Yep, I had it on the dyno and confirmed that it is blazingly fast. It's getting 55 HP to the rear wheels !”, or “That's nothing, you should see the monster that Fred is building. It's not finished yet because he needs a rear end from a Euclid to handle the power“, or Just last week on the rally, I blew the doors off Mike's 2CV !”, or “After breakfast, I blitzed the quarter mile. Then we had lunch !”. There are ooohs and aaaahs and nods of admiration for those that have the fastest version of an old machine.
The fourth answer is because you can. More is better. It's basic math. This often comes about from sitting around in a garage drinking, and begins with sentences like "What are you gonna do with that old...?" or "What would Chapman do?", or "You know what would be freakin outrageous.. ?", or "I bet you could fit a turbo right here..", or "Did you just dare me to do that ?".
The fifth and final answer is that we have an innate need to go faster. Faster than stock, faster than the next guy or gal, faster than the last run, just faster. The guys and gals at the extremes (“I'm telling you Nick, with a little fab work that M5 engine drops right into the Isetta engine bay, I measured it.”) are just the rest of us with more imagination, or zeal, or money, or screws loose. I have always thought it would be cool to have a bone stock properly tuned example of all of the vehicles that I have owned, and then have another that could be tastefully and subtly modified, and then a third that would be the outback steakhouse version (no rules, just right). This way my inner selves could all have fun.