Monoflow to Series loop conversion

I have a two-story Cape Cod house with a monoflow heating system. There is a single 1-1/4" black pipe main down the center of the basement, which splits at the end into two 3/4" branches, one for each side of the house. There are monoflow tees that feed the 1st & 2nd floors. Off of each Tees are 1/2" risers to each of the Rads. I want to separate the 7 Rads on the 2nd floor into it's own zone, using it’s own flow control valve and either zone valves or another B&S pump.
1)- Can I change the 2nd floor into to a series loop system by itself (not Monoflow) if I provide its own separate main, pump (or zone valve), flow-control valve and return?
2) - If it has to be another Monoflow branch for the 2nd floor, can I use 3/4" piping for the main feed and the return? Will I disrupt the balance of the entire system by splitting off the 2nd floor, using the existing pump, two flow control valves and two zone valves (one for each floor)?

monoflo

I find it hard to visualize how a 3/4" main with 1/2" MonoFlo tees would be able to service sever radiators on the second floor with any kind of an acceptable comfort level. The problem with converting it to a loop system, unless you repiped the entire system, is that once you connected to the first set of 1/2" risers, the rest of the system would be "starved" due to the flow restriction it would cause. Personally, I think you need a heating calculation and a redesigned installation. Your water velocity rates in the pipes must be huge if it is working properly.

Monoflow to Series loop conversion - Correction

I made an error. Last night I rechecked the pipe sizes. The main looks like at lease 1-1/2", which splits off into two 1" pipe sub-mains. Each of those 1" sub-mains do have monoflow Tees with 1/2" risers to the Rads.
If I was to split the 2nd floor off onto itself, there will only be be 7 rads. Could I use a 3/4" main, feeding the existing 1/2" risers to the 2nd floor?

loop

Not likely. You would probably have to maintain the 1" main, and given your piping system a loop system would be impractical.