You have several options:
- Use a guitar pedal (a compressor, or a boost, or any pedal that has a volume control that gives you a boost over unity should work, a multieffects device should also work).
- Build something like a stompbox adapter module (see also this version, and a PCB version) that you can use it to boost the signal.
- Build a simple booster for guitar. You can use an op amp, but even a simple transistor design should do as in the classic LBP-1 (stripboard layout here).
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