If you are only connecting one emonTx via serial USB, you wouldn’t need to write udev rules, if you think you may connect more emonTx(s) via serial USB I would still recommend using an FTDI based adapter just in case (saves ditching a non-ftdi one when you hook up a second device).
There are several threads on “serial-direct” connected emonTx and this “Does the emonTx Shield need an actual Arduino? - #21 by pb66” thread might give you some info on sourcing and connecting a ftdi usb serial adapter.
[edit] Since the emonTX RFM is possibly stalling things you may need to edit the sketch to remove the RFM bits so that it doesn’t stop when using a serial adapter, in which case you can edit the sketch to send the node id as part of the data packet, so you could get away with using an OEM shop sold USB adapter.
if you get an adapter (and leads?0 we can walk you through the changes, it’s not that difficult.