I have built two of these things for different grandchildren, completely out of wood and from a found trashed edger mower. With a syntrifigal clutch, they are easy to make.
I have always wanted to build some sort of effective, yet safe pedal accelerator system, but just have not felt comfortable in its retraction or de acceleration. Springs have to bring the pedal back to idle mode effectively or you will have problems.
I have always fallen back to the basic lever system that is mounted right by the seat that lets the driver go from faster to slower just by moving the lever. No springs, no moving parts actually except for th inner cable. Then I have an extension to the driver for the spark plug arrestor kill lever also.
That pedal system is great, but I do not think you can competely trust it.