S&T m1903 upgrade
 

Is there anyway to upgrade the s&t m1903? or to converter other upgraded parts for it?

Some parts are VSR compatible. Upgrading is possible, though limited. What are you trying to achieve?

well, I hope for a better hop up, and the american m1903 is at about 370 fps so it's pretty lame for a sniper, I hope to get better fps too.

The Hop-Up rubber is VSR compatible. So you can get a good rubber and it will help with accuracy. If your stock rubber doesn't seal well, FPS might go up too.

As for power, you can change the spring, but frankly, 370fps is plenty for a rifle of this type. It allows you to engage from closer since you have the same MED (minimum engagement distance) as everyone else, that's really useful if you play in heavy woodland where close engagement is frequent.

what kind of spring you suggest me to buy?

Any vsr10 spring should do. But don't expect your effective range to increase significantly from a spring change. The hop-up rubber has a much bigger impact on your range than the spring.