Play midi file through different instrument

Yes.
That is part of the beauty of MIDI.
Your MIDI track output can be directed to any softsynth you have inserted in your project.
It's just note information, and it generally doesn't care where it is being sent.
The only caveat is downloaded GM files, which may have patch data embedded.

If you are using the normal template, it loads the synth as an Instrument Track. While there is nothing really wrong with this, the MIDI track is locked to the synth audio output. You can't see it, but an Instrument Track is really two tracks integrated into one.
To manipulate the MIDI track output, it must be isolated from the synth.
Don't worry, it's a fairly easy fix.
Going forward, I would recommend inserting your synths and selecting the following options:
MIDI Source Track
First Synth Audio Output (or All Synth Audio Outputs if the synth supports multiple outputs.
Synth Properties Page

Do you have Kontact inserted as a synth?
If so, insert a new MIDI track. Copy/paste the MIDI data from the SI track to that, or drag the SI patterns from the SI GUI to the new MIDI track.
Set the output of this MIDI track to Kontact ( or any synth)
At this point, the pattern should play.
If you get hung up somewhere, let us know where you got stuck.
The key is keeping the MIDI track independent from the synth.

Just to say that although the method above worked in that case, I found it doesn't always work. I had some other problems but it was complicated to follow on from this post so I started another 
Midi track out of sync with others