Here's all the particulars:
https://ratrodbikes.com/threads/rrbbo-19-rules.119662/
And here's all the builds in the current build off:
https://ratrodbikes.com/forums/%E2%AD%90%EF%B8%8F-rrbbo-19-build-journals-%E2%AD%90%EF%B8%8F.282/
Check it out, it's tons of fun!