To calculate force at exhaust port specifically is of little use, but nevertheless, it is possible.
Honda GX200 specs - Max power 6.5HP@3600 rpm or 60 rps. We will calculate thrust at this rps.
196 cc engine. The exhaust port diameter from diagrams is estimated to be around 2.8cm.
Assuming complete combustion of exhaust gases we will use the density of air at 1000k as a substitute as the density is subject to large variation due to active air-fuel ratio, load conditions etc. Density = .3716kg/m^3
Engine is pumping .196L of exhaust gases every 2 revolutions as it is a 4 stroke engine.
Therefore at 60 rps, it is pumping .196L 30 times every second. That is equal to 5.88L/s or .00588m^3/s. This is the volumetric flow rate.
The cross sectional area of the exhaust port is pi*(d/2)^2 = .0006157m^2
Therefore the Exhaust gas velocity is .00588/.0006157 = 9.54m/s
We can now calculate thrust or force by using the following equation
F=density* Me* Ve =.3716*.00588*9.54 = .020 N of force. This is negligible and not enough to force the engine forward r in any particular direction, but can be used as an effective blower or pump of sorts with appropriate tubing. The only way to increase thrust at this rpm would be to restrict the exhaust manifold by reducing diameter and increasing length. This would cause the exhaust gases to be forced through a smaller space and hence will develop the tendency to escape quicker, ultimately increasing exhaust velocity. Hope this answer helped.