You won't get the full power of the machine unless it is running on full throttle, so run it on 3/4 by all means, but be prepared for the drop in performance.
Have you got a pressure guage on it... if not you can get one for a few quid and you will see the difference in what throttle you use.
Regarding the whirlaway, just check out ebay, they have some fairly good deals on there. You probably won't find a 2nd hand one as people tend to only get rid of them when they are knackered, but you will find a few new ones on there.
Cheers
Andy