I highly doubt it will break 140bhp. The cam is a big unknown, it has most probably got a Kent BCF2 or Piper 285 as these don't need valve cutouts in the pistons.

The CR will probably be around 9.5-10:1 than is achieved by using 1.3GT pistons. Although I haven't heard of 1680cc, 90+ thou pistons give 1693cc.

So if it has these mods a small amount of head work, or none, and twin 40's I would realistically say 100-120bhp if properly set up. Check the choke size on the webers, don't be tempted to go any higher than 34mm on that engine.