The Curare with a 90LX would move rather well..... but again, probably in that 130 mph top end speed range still... but its a fairly big plane and will look like its moving
That is just a guestimate based on what my old Tipo, Curare flew like years ago with (tweeked) Rossi and OS power up front.
If you are just sport flying the Vipers....... stick with them for some speed.
1) take your existing engine and bolt on a jett-stream muffler --- move to a 8.75x8W or 9x7 prop. That will get you to the higher 130s.
2) Viper with TT46pro- jett-stream muffler ----- just a bit faster, not much
3) There are a lot of used Q-500 engines on the market for a good price. Both Nelson and Jett 428 engines. Bolt one of those on the front of the Viper with a 8.8x8.75 prop. Instant 150mph + aircraft.
4) If you want similar performance to 428 but want a throttle ---- Jett 60LX engine.
Also,
it depends in whether you want to build a plane... lots of potential pattern ship projects out there, and a few decent prop-jet and dedicated speed designs too.