That would be possible, but would require making separate tech areas for each engine type. And even then, you'd have to account for the different tech cost settings - on Low tech cost, if level 1 ion engines are 5000 RP, level 3 ion engines are 15000 RP, but on High, they would be 45000 RP! So you'd either have an abnormally large gap in low-tech-cost games, or an abnormally small gap in high-tech-cost games.
