Are there any open source alternatives to Chainalysis Reactor for doing forensic analysis of a local copy of the full Bitcoin blockchain?
No I have not seen any open source projects similar to Chainalysis Reactor. Even explorers like OXT that are helpful in on-chain analysis are not open source.
It will be a great project if someone can create such open source tools because sometimes you need to break things to improve them. Ignoring tools used by companies like Chainalysis doesn't improve anything.
BlockSci might be relevant, though "As of November 2020", it "is no longer actively maintained".
Abstract of a paper by one of its devs:
Analysis of blockchain data is useful for both scientific research and commercial applications. We present BlockSci, an open-source software platform for blockchain analysis. BlockSci is versatile in its support for different blockchains and analysis tasks. It incorporates an in-memory, analytical (rather than transactional) database, making it orders of magnitudes faster than using general-purpose graph databases. We describe BlockSci's design and present four analyses that illustrate its capabilities, shedding light on the security, privacy, and economics of cryptocurrencies.