The problem is not so much building as it is cooling small engines. Scaling works against you here. You can download a good simulating tool for liquid fuel rock

The problem is not so much building as it is cooling small engines. Scaling works against you here. You can download a good simulating tool for liquid fuel rocket engines - http://www.propulsion-analysis.com/ http://www.propulsion-analysis.com/ - it will show how, when you're decreasing engine size and fuel flow it becomes not enough fuel to cool of the engine. Spike nozzle is particularly problematic around the throat, which has bigger area than for a regular bell-shaped nozzle.