Engine Management

MegaSquirt

https://www.diyautotune.com/

Quite possibly the most popular "budget" after market engine management. There's multiple different models to choose from which should cover most use cases for engine management. Lots of documentation is available and customer support as well.

Speedunio

https://speeduino.com/

Speeduino is a VERY cheap after market solution that works great. It does have some limitations, such as it can only run four cylinders with sequential spark and fuel.

rusEfi

https://rusefi.com

rusEfi, like Speeduino is a very cheap engine management. It has less limitations than the Speeduino but is not a very well known platform with limited documentation.