#define MAX_PLAYER_ATTACHED_OBJECTS 5 // This is the number of attached indexes available ie 5 = 0-4
native SetPlayerAttachedObject(playerid, index, modelid, bone, Float:fOffsetX = 0.0, Float:fOffsetY = 0.0, Float:fOffsetZ = 0.0, Float:fRotX = 0.0, Float:fRotY = 0.0, Float:fRotZ = 0.0, Float:fScaleX = 1.0, Float:fScaleY = 1.0, Float:fScaleZ = 1.0);
native RemovePlayerAttachedObject(playerid, index);
native IsPlayerAttachedObjectSlotUsed(playerid, index);

#define SetPlayerHoldingObject(%1,%2,%3,%4,%5,%6,%7,%8,%9) SetPlayerAttachedObject(%1,MAX_PLAYER_ATTACHED_OBJECTS-1,%2,%3,%4,%5,%6,%7,%8,%9)
#define StopPlayerHoldingObject(%1) RemovePlayerAttachedObject(%1,MAX_PLAYER_ATTACHED_OBJECTS-1)
#define IsPlayerHoldingObject(%1) IsPlayerAttachedObjectSlotUsed(%1,MAX_PLAYER_ATTACHED_OBJECTS-1);

#define VEHICLE_PARAMS_UNSET -1
#define VEHICLE_PARAMS_OFF 0
#define VEHICLE_PARAMS_ON 1

native SetVehicleParamsEx(vehicleid, engine, lights, alarm, doors, bonnet, boot, objective);
native GetVehicleParamsEx(vehicleid, &engine, &lights, &alarm, &doors, &bonnet, &boot, &objective);

new engine,lights,alarm,doors,bonnet,boot,objective;

if(strcmp(cmd, "/startengine",true) == 0) {
new vid = GetPlayerVehicleID(playerid);
if(vid != INVALID_VEHICLE_ID) {
GetVehicleParamsEx(vid,engine,lights,alarm,doors,bonnet,boot,objective);
SetVehicleParamsEx(vid,VEHICLE_PARAMS_ON,lights,alarm,doors,bonnet,boot,objective);
}
return 1;
}
if(strcmp(cmd, "/stopengine",true) == 0) {
new vid = GetPlayerVehicleID(playerid);
if(vid != INVALID_VEHICLE_ID) {
GetVehicleParamsEx(vid,engine,lights,alarm,doors,bonnet,boot,objective);
SetVehicleParamsEx(vid,VEHICLE_PARAMS_OFF,lights,alarm,doors,bonnet,boot,objective);
}
return 1;
}

Code
SetVehicleEngineOn(vehicleid)
{
new engine,lights,alarm,doors,bonnet,boot,objective;
GetVehicleParamsEx(vehicleid,engine,lights,alarm,doors,bonnet,boot,objective);
SetVehicleParamsEx(vehicleid,VEHICLE_PARAMS_ON,lights,alarm,doors,bonnet,boot,objective);
}