Dislcaimer: I fully support 64-bit and think it was a bold but wise decision for Apple to move Logic to 64-bit only. However I also sympathize with users of Sylenth1, which is a staple for most dance music producers and as developer has more or less fallen off the face of the earth, there is likely no chance it will ever be upgraded to 64-bit.
As such, here are 3 ways you can use Sylenth1 with LPX:
1. Use Native Instruments' Maschine to host the VST version of Sylenth, then jBridgeM for 32-bit support (tested, works like a charm)
2. Vienna Ensemble Pro (have not tested, but told it works)
3. Run LP9 alongside LX and print audio
Hope this helps somebody