the words "Speed" and "Servicar" should not be used together in the same sentence, but you can get them to run a little faster on the highway.
Your best carburetor choice is the Linkert DC-2 that the factory installed on them from 1959 to 1965. Watch carefully when buying one on ePay or other web sites because the DC that fit the 45 flathead engine was "left-handed", the float is on the opposite side of the DCs they put on the big twins.
The servis usually came with a 23 tooth engine sprocket, for more top end you can install a 27 tooth and you may even get by with a 29 tooth if you normally drive where the roads are flat. When installing the larger engine sprocket it is sometimes neccessary to also install a 100 link primary chain so you stay within the adjustment range of the slots under the transmission.
My '71 servi is quite happy at 55 mph, all day long.