There are a couple of really good reasons why classic cars and bikes never make the origianl claimed power:
Original claimed power figures were often pure fiction - read any books about the industy and it's clear that the procedure 'often' followed the 'look at your closest competitor and add 10% to their claims' method. Good for sales and bragging down the pub, but that's about it
Press demonstators were often specially fettled or tuned to provide much more power than the machine available to Joe Public. Suzuki and Ford (I'm thinking of the RG500 stroker and the some of the hot Capris) were particularly good at this, as were Ducati and Guzzi.
Also, until camparitively recently, hardly anyone outside the industry had access to a dyno - Bike Magazine did, but it was an ancient Heenan and Froude water brake that constantly overread by up to 20%.
Also, each dyno gave a different reading: If manufacturers did test, they would make sure it was on the dyno that gave the highest reading. Dyno's are great at comparing engine power curves, but unless calibrated properly, useless at giving accurate max power readings.
You can however build a modern replica of an old engine with modern manufacturing materials, techniques and tolerances that will produce way more power than an original - some companies (Works Replica for one) now make Manx Nortons that pump out nearly 75hp, way, way more than an original, built using worn out pre WWI tooling could ever hope to achieve.
Anyway, everyone knows it's not about power, it's about how you use it.
That's why Mini Coopers and Saab's kicked the ruled World Rallying for over a decade