The steel receiver 60 could be improved a little with an o-ring on the pellet probe and an extended spring guide. You may be able to find Russ's tune on- line somewhere. The cocking mechanism and arm are not strong enough for a heavier spring.
If you are lucky, you might get 500 fps out of them. A guy was trying to sell one a year or so ago and he said it was doing over 700 fps. He listed a long string of things he said he had done to it. But, on further investigation and probing him with questions, he was using WD40 as a "power booster".
The neatest thing I have seen done to one is that Theodore Summers had Steve Corcoran make a nice laminated stock for one. What a pretty little rifle.
David Enoch