I would differ slightly from the above posters, especially if you can avoid checking bags.
In the evening there are fewer delays at SFO, because any weather mostly affects daytime rush hour flights when airport capacity is maxed out. So if you accept the average likelihood of your inbound flight being on time, 1:45 would be enough time.
To walk from the United "normal" gates to international terminal is about 10 minutes. By then you are at check-in. Security at intl is never more than 10 minutes, 15 at most if there is a rush. If you could print your BP in advance you wouldn't even need to leave secured area.
If you have bags then allow 20 more minutes.
I would say it depends on how confident / risk-averse you are, whether you have bags. Although the 2 separate tickets does make it more troublesome in the rare likelihood of severe delays.
The bolded part above is incorrect with respect to San Francisco. SFO's delays, when they happen, occur as a result of low clouds or fog closing one of the two active
arrival runways. When that happens, morning flights are almost always fine because so many aircraft arrive the night before and overnight at SFO. The delays start to build in the mid-morning, getting worse all day until they peak late in the evening. It's not uncommon for morning departures to go out on time or 30 minutes late, whereas 8 p.m. departures are two hours delayed.
The other thing to keep in mind is that the long-haul overwater flights are often prioritized over the domestic flights, meaning the OP's MCO flight might be delayed by two hours or more, while the SQ flight goes out with little or no delay.