Unfortunately, I don't have a receiver that covers 300MHz.
Essentially, that's what my oscillator+mixer+amp setup would be -
an ad-hoc receiver.

I just had a couple new ideas: Wonder where my old grid dip
meter got off to? Think it's in the garage. Would it be accurate
enough? Probably not....

Might be the excuse I've been looking for to buy an old HP
spectrum analyzer .

I just traced the thing's circuit. I think I've identified a node
that
can be grounded to the battery to make the thing emit continuous
wave RF. Then just belly it up to the frequency counter....