I've done this while I should be working so it's rough, but here are some thoughts.
Let's evaluate the Same Sex marriage free vote potential. I'm going to take worst-case scenario for Tories and best case for NDP and for Bloc. I figure that the few Tories who vote to maintain it will be cancelled out by the few NDP/Blocheads who go against their party lines.
That means, absent Liberals, we have 124 against, and 80 for.
With the Liberals, last time around, 37 voted against SSM (or were absent or paired). Many of them were re-elected. By my count, 28 of those 37 were re-elected (only Carr, Gallaway, Lastweka, Longfield, Savoy, and Ur lost).
So we have 28 guaranteed Liberal votes against SSM (or for a Harper motion repealing). Now that there is no cabinet, it's anyone's guess how they fall. I'm going to guess that the remaining Liberals vote in favour, since the incumbents did last time and no one knows how new candidates will go. That means there are 75 Liberal votes for SSM.
Thus, my simple, simple math gets us to this calcuation:
Against SSM: 124 Tories and 28 Liberals = 152
For SSM: 51 Bloc + 29 NDP + 75 Liberals = 155
Basically, it's close. Very close.
Two calculations then come into play. First, how many Liberals who voted against SSM last time will decide either a) the issue has been put to bed, let's just leave it; or b) decide that it will help the Liberals to portray the Tories as socially conservative barbarians by swallowing their objections and voting to uphold SSM.
Second, given how close it is, will Harper decide to just let this issue slide in order to build his government and focus on other priorities, not wanting Canadians to get an early impression that he has a social agenda.
At the same time, Harper could also profit from ramming his motion through now while the Liberals are headless, rather than face the issue later in his Parliament when the opposition will be less inclined to make it work.