The most reliable sources for the RX 135 (and the RX-G/RX-Z variants) are enthusiast forums and document sharing sites. You will likely find it under "Yamaha RX 100/135 Workshop Manual" as the chassis and engine architecture are similar.
Stock is 18° BTDC at idle, advancing with RPM. The manual shows you how to use a timing light and where the marks are on the magneto rotor. If you fit an aftermarket CDI, you set it back to stock first, then adjust. yamaha rx 135 service manual
