Problem is 1.1 and probably Murnatan are both very different from TF2.
Instead of one builder with a single turret, you have many, many turrets with base defenders mixed in.
Historically it's usually about 65/35 as to who gets credited for the kill, in favor of the defenders.
Tracking the separate damage dealt by each player and their structures would be a much more accurate representation of usefulness on a team. 100 damage dealt or 1 frag (tracked separately) should award 1 score point. Evos and credits should probably still be based on frags alone though, to keep people from farming hallways or enemy bases for credits by spamming turrets, etc.
Builders might also get credit for keeping their structures alive. For instance, awarding 1 score point for every 100 damage repaired from a structure, and taking 1 score point away for every 100 damage dealt to a friendly structure.
A seperate Elo rating system could be fun too:
http://en.wikipedia.org/wiki/Elo_rating_systemUsually pretty accurate.