The Magen Avraham (OC 428 sk 8) records a custom to give a Levi the Aliyah containing the story of the Golden Calf. Indeed in many Chumashim nowadays I see the first two portions of Ki Tisa are exceptionally long so a Levi can read that section.
What are the original Aliyah divisions for the Parsha which were used before that custom caught on? Do we not know? Is that custom so old that it precedes published Chumashim? This might be useful in case there are no Kohanim or Leviyim present.
Indeed the custom you mention is first mentioned in 16th century Safed in the work Tikkun Yissakhar by Yissakhar ben Mordechai, and while he notes with praise the custom to have a Levi read the section of the Golden Calf, he mentions two older common customs for how to divide the Parsha.
One custom he records is to have breaks at 30:11, 30:30, 31:12, 32:15, 33:12, 34:1, 34:27.
The other custom he records is to have them at 30:11, 30:22, 31:12, 33:12, 33:17, 34:9, 34:27.
In this Chumash from 1580 there are breaks at 30:11, 30:22, 31:12, 32:15, 32:30, 33:17, 34:27.
Yemenites traditionally have breaks at 30:11, 31:12, 31:18, 32:15, 33:12, 33:17, 34:1.
In this old list of customs from Fürth as well as in the modern Yekkish custom the breaks are 30:11, 30:22, 31:12, 32:15, 32:30, 34:10, 34:27.
(This variety is not surprising given the general variety of Aliyah breaks in different communities.)
Probably any of these are fine if you have no Levi or Kohein around (or you are German or Yemenite or otherwise don't have that custom the Tikkun Yissakhar mentioned).
