DURING CRUISE FLT WITH THE STUDENT PLT FLYING THE ACFT, THE ENG EXPERIENCED A PARTIAL LOSS OF POWER. THE CFI TOOK CONTROL OF THE ACFT & WAS ADVISED BY ATC THAT A GRASS AIRSTRIP WAS BELOW. THE FLT CIRCLED WHILE DESCENDING & THE CFI ASKED ON THE UNICOM FREQUENCY IF ANYONE WAS MONITORING IT BUT RECEIVED NO REPLY. WHILE CIRCLING HE OBSERVED AN ACFT TAKE OFF TO THE SOUTH & ELECTED TO LAND IN THAT DIRECTION. WHILE ON A MODIFIED FINAL APCH, HE OBSERVED AN ACFT ON THE APCH END OF THE RWY. AFTER LANDING APRX 1/2 WAY DOWN THE 2,300 FT GRASS/SAND RWY, THE ACFT TRAVELLED OFF THE RT SIDE & COLLIDED WITH A TREE. EXAM OF THE ENG REVEALED THAT THE NO. 2 CYLINDER EXHAUST VALVE FAILED NEAR THE RADIUS OF THE SEAT. NO DETERMINATION COULD BE MADE AS TO THE REASON FOR THE FAILURE. THE ENG HAD ACCUMULATED 1,840 HRS SINCE MAJOR OVERHAUL.