There is no program besides Jonathan Forbes' LZX itself that can compress LZX archives. No PPC version exists, no way to create LZX archives natively on any other platform.
There are several LZX decompressors available, I co-authored one of them. Of course you can make guesses about the algorithms used when looking at the decompression code. You could also learn from the disassembled executable, but given the complexity of a compression method that uses several algorithms (Lempel-Ziv, Huffman, arithmetic coding) that disassembly isn't going to yield any actionable insights.
Micro$oft may have improved the compression by using dynamic and larger search distance, but the Amiga LZX method can still be reproduced using the CHM/CAB compression scheme with few modifications due to the different endianness.
Using that leaked code is of course impossible for legal reasons. But it could serve as a draft for someone to reimplement the algorithm in ar002 (LZH) - the grandfather of most archivers used in the 80s.