Originally Posted by Andy_K

For starters you'll need a clamp-on bottom bracket shell like this one. You probably need to cold-set the spacing on your front wheel (if your penny farthing is aluminum or carbon, just forget it) and rebuild your front wheel. I'd recommend a flip-flop hub so you can coast when you want to. You'll probably need a tandem chain and a seat post with a lot of set back.

Good luck with your project.