I think its an interesting question, and a bit situational. There's already a fair number of factory-turbocharged cars that are recognized classics, such as the Corvair, 2002, Porsche 930.
And the timeframe for "classic" is rapidly moving into the 70s and 80s, when turbocharging really took off.
On the other hand, there's a fair number of classics that seem utterly unsuited to forced induction.
If you expand "turbocharging" to include supercharging, it gets even murkier. Bentley and MG, for example, were famous for their supercharged cars way back in the 30s, and there's some very nice supercharger kits available now that provide very modern performance in a weekend.
For me, I've always wondered about the MGB. I know there were some turbo kits sold for the B back in 70s, and the B engine is a tough beast. On the other hand, the B head is prone to hotspots. I think it would be an interesting turbo experiment, but i'm not sure it would work.
