The 987c Si (200/50kHz) uses a different set of 2D sonar frequencies than does the 1198c Si (200/83kHz), but at higher boat speeds you won’t be using the 50kHz or 83kHz so I would just go ahead and but the one for the 1198c Si unit now and just not worry about using the 50kHz on the 987c Si unit.
XP-9-20 DualBeam inside-the-hull transducer
Or
XNT-9-20 DualBeam transom mount.
You will also need the AS-Si-DB-Y Side Imaging - DualBeam splitter cable or the TS3 Transducer Switch.