I think your speed is reasonable. It have gotten yourself a fine scoot. Looks like you have a much better handle on mechanical then I. On my Helix 1986 the intake manifold cracked changing the mixture and blowing a head gasket. seems it is typical for the older machines. a replacement would have saved me so much cost and time as I ended up replacing the engine.
It would be good to go to a site on the web and see what problems have popped up for others with elite 150's. Have fun with your new scoot. It is an antique.