how do i manually construct a scala runtime ast that instantiates a class?

Besides extractor’s apply metod accepting a Name

abstract class IdentExtractor {
  def apply(name: Name): Ident
  //...
}

https://github.com/scala/scala/blob/2.13.x/src/reflect/scala/reflect/api/Trees.scala#L1787

there is also factory method accepting a Symbol

def Ident(sym: Symbol): Ident

https://github.com/scala/scala/blob/2.13.x/src/reflect/scala/reflect/api/Trees.scala#L2237

Try

val actualTree = Apply(
  Select(
    New(Ident(typeOf[Foo].typeSymbol)),
    termNames.CONSTRUCTOR
  ),
  List(
    Literal(Constant(1))
  )
)

println(ru.showRaw(actualTree)) 
//Apply(Select(New(Ident(my.package.Foo)), termNames.CONSTRUCTOR), List(Literal(Constant(1))))

println(toolbox.typecheck(actualTree).tpe) // my.package.Foo

Quasiquotes and reify/splice are preferable ways to construct trees. So you should provide more details why q"new my.package.Foo(1)" and ru.reify { new Foo(1) }.tree are not enough for your use case. Normally it’s not necessary to build identical trees, it’s enough to build trees that behave as desired (but there are exceptions).