I developed and build a very simple converter 739 MHz to 145 MHz with just 3 Transistors and one standard XO.
It works excellent for receiving Oscar100 NB transponder with a 2m RX.
I wrote a complete report and building instruction for it.
You can find it at : https://elektronik-muenster.de…ic.php?f=17&t=33&p=85#p85
Sorry, german language only.
But there are lots of pictures an shematics also for download so everyone should it find interesting.
In other case "deepl" helps