Evidence Act

I'm an advocate practicing in Chennai. I would like an advise from our learned members in respect of the following issues:

1. If a document is required to be registered by law , but is not registered by the parties to it, can it be introduced in evidence? If so under what provisions?

2. If a document is required to be stamped, but is insufficently stamped or not stamped at all, can it be introduced in evidence? if so under what provisions?