You could move to the emonTx Shield and an Ethernet-equipped Arduino, or a plain Arduino with both Ethernet Shield and emonTx Shield. I’m not an Arduino expert, the Shield and its software are designed for the Arduino Uno, so I’d imagine that most Arduinos that have Ethernet and are Uno-compatible will be usable - but note the warning in the Wiki about compatibility. If you’re using an Ethernet Shield, bear in mind the emonTx Shield only stacks as the top one. And make sure you solder the connectors on the correct side of the p.c.b.
Or, have you seen this: Simplified emonTx Arduino Shield + Ethernet Shield - Guide