You can't compare a vertical engine to a horizontal in either two OR four stroke ... due to two main factors ..... vertical engines have straighter intake and exhaust gas flow paths which give them a big advantage .......

Secondly ..... the piston doesn't have to deal with as severe windage losses in a vertical design ........ whereas the poor old horizontal 4 stroke has to constantly deal with oil drag and having to push oil out of the way all the time ...... that's one of the reasons they suffer from crank case pressurization problems when you rev them hard .......

Thirdly .... a lot of vertical china engines have push rods instead of an overhead cam and timing chain ...... so they have to deal with higher inertia of the valve train components limiting their peak revving ability ....... Jap engines are all over head cam these days ...... and the newer MX engines such as the CRF150R go one step further than standard OHC engines by running 4 valve heads ..... some with 5 light weight titanium valves and cam buckets to eliminate the weight and inertia of rocker arms and normal stainless steel or carbon steel valves ........

Formula1 cars go one step further and eliminate the final weak point ..... VALVE SPRINGS ....... they run air controlled valves and that allows them to rev reliably to well over 20,000 rpm ......