While an interesting challenge, I'd be inclined to leave the uBitx alone and build a stand-alone AM transmitter/receiver.?
Or at least an AM transmitter that you can use with your uBitx. The uBitx as an SSB receiver will receive AM but you will need to zero beat stations. AM operators aren't necessarily on the same frequency so if there's a group on you may need to slightly adjust the tuning if trying to receive AM on an SSB receiver. Whereas with a dedicated AM receiver you don't have to.?
If you were to build an AM receiver I'd use something like an NE602 for the converter, an MK484/ZN414 as an IF amplifier and detector, some 455 kHz IF transformers (or ceramic filter) for IF selectivity and an LM386 for audio.?
As for an AM transmitter there are 7 MHz pirate radio designs of about 10w that can be adapted for amateur use.?
I discuss AM in more detail at?http://home.alphalink.com.au/~parkerp/gateway/am.htm
73, Peter VK3YE
Peter Parker