Unfortunately, there's absolutely no cheap way to increase hp on the M117.968. You have to install 10:1 Euro pistons to get the 300hp. All other forms of modifications are pointless. First gear start on the transmissions make a nice addition. A 2.82 or 3.06 rear axle helps substancially as well.
An exchange Euro engine is around $6500 from Metrics with a 4yr/50K warranty.