Boss Cowan continues to shape his squad for the 2021/22 campaign and has agreed extensions with long-serving defender Lilly and young attacker Bower.
Lilly, 22, will go into a seventh season in the first-team picture at the New Bucks Head after making his debut in the 2015/16 season.
He started at Telford as a forward but has in more recent years been converted into a centre-half by Cowan, who himself was a former central defender.
Lilly started in seven of the Bucks' 17 league games last season and scored once, in the 2-0 home victory over Guiseley in late November.
The boss said last season that the young defender has all the tools to succeed with the Bucks but challenged him to find consistency.
Cowan said at the time: "If Zak wants to be stronger than someone he will, if he wants to jump higher than someone he will, if he wants to run quicker than someone he will, and he can play.
“It’s consistency...my thing with him is he can't have a game off."
Also agreeing new terms is pacy attacking prospect Bower, 20, who Telford snapped up from The New Saints last summer.
Bower, from Shrewsbury, was used just once from the bench in league action last term.
A winger who can also operate more centrally, Bower impressed manager Cowan as a trialist last summer.