\input{luaotfload.sty}
\directlua {
  local function patch(fontdata)
    local mc = fontdata.MathConstants
    local em = fontdata.parameters.units
    local sz = fontdata.parameters.size
    if fontdata.psname == "CambriaMath" and mc then
      mc.DisplayOperatorMinHeight = 2800 / em * sz
    end
  end
  %% part of luaotfload-auxiliary
  %luatexbase.add_to_callback(
    %"luaotfload.patch_font",
    %patch,
    %"luaotfload.aux.patch_cambria_domh")
}

\font\4={name:Cambria Math:mode=base;script=math} at 10pt
\font\5={name:Cambria Math:mode=base;script=math;ssty=1} at 7pt
\font\6={name:Cambria Math:mode=node;script=math;ssty=2} at 5pt
\textfont4=\4 \scriptfont4=\5 \scriptscriptfont4=\6

$$
\Umathchar"1"4`∫
\Umathchar"1"4`∑
$$
\bye