When considering use of FT-243 and other vintage crystals today, it is important to remember that many of them are compromised by self-contamination, corrosion, and or even just dirt at this point.  In my experience many of these crystals have failed entirely or are barely working at all (unreliable, unstable, or low activity) - even without trying to run them in a power oscillator!

After opening and rebuilding hundreds and hundreds of FT-243s over the past several years, I've very frequently observed all these problems. It is rare to find an FT-243 that does not suffer from any of these issues.  My theory about the contamination is that the rubber seals and other materials in the crystal holder have outgassed material that was deposited on the quartz and electrodes.  

Also, the seals on many crystals long ago failed and have allowed foreign material and adverse environmental conditions to reach the innards.

Fortunately, none of this significantly bothers my work to place modern, miniature crystals inside the vintage holders.  The modern units are hermetically sealed on their own.