There are really three possible options here?.
1) You're a radio amateur, and you've converted it to an amateur band. In which case you should know hwo to calculate and order crystals. To make it fully adjustable would require a frequency synthesiser, specifically engineered for the Cambridge - but it's not a trivial task.
2) You're a legal and licenced user, in which case you're only allowed to use the frequency you already have!.
3) You're an illegal user, and you shouldn't be messing with radio transmitters!.