My 10Mhz TCXO that I use for a frequency counter timebase has gone
toes up and I only have a 5Mhz one available to replace it. Can anyone
offer ideas to double the 5 Mhz TCXO keeping the M:S ratio accurate
enough for use with the frequency counter ?.

Feed the 5 MHz to both inputs of a high level DBM. The output will be sum
and difference, kill the DC component with a 0.1 uF coupling cap and you
probably can use the 10 MHz result. If not, a little low pass filter to get
rid of the higher frequency responses will make it useable.

W4ZCB