Can i put old aqua stat in new indirect water heater or is it advisable to replace it is same model water heater but their aqua stat # is l4080B-1352

Hello, I am looking for some assistance on some alterations we are making to a current HW baseboard system. To give some background the home is a recently bought (was bank owned) 2-family in North NJ, approx 3,000 sq ft. and has one gas HW boiler. Here are some additional details: -2 zones (1st floor unit and 2nd floor unit) -1st floor zone is a monoflow setup 1 1/4" main pipe that splits one way to supply two slant find baseboards in the front of the house (1st unit), and another way to supply 5 baseboards and a kickspace heater in back of the house (1st unit). There also used to be a few branches coming off the main in this system to supply some baseboard on the 2nd floor - but these were removed. In total it has approximately 62' of slant fin The 1st floor loop/zone works great and provides very good/even heating to the 1st unit. -2nd floor was a mixed bag: there was a series loop supplying a few rooms and the other rooms were included in the 1st unit zone and supplied by the monoflo setup. -Has a Taco-007 circulator on the return pipe just before entering the boiler. -Peerless boiler We are doing some renovations and would like to put all baseboards on the second floor on the series loop. The concern I have is that the series loop never got hot, which I think may just bee a ton of air in the loop (we hear some gurgling and slushing of water when calling for heat). Details about the series loop: The series loop is a 3/4" copper loop that originates from a standard T off the main 1 1/4" supply pipe just after the expansion tank. It the routes itself up to the second floor and follows the perimeter of a portion of the unit and comes back down into the basement and meets the zone valve before connecting back into the main 1 1/4" return pipe. Currently it is about 200' in length and has approximately 36' of slant fin baseboard. We want to add approximately 22' more slant fin to this loop to keep the entire second unit on its own loop/zone (essentially joining the 2nd floor baseboards that were on the monoflo system previously to this loop) and total length of the loop would be approx 245'. My concern is we are adding to a series loop that currently does not get hot water (although I am positive it has worked in the past, there were clearly people living in that section of the home previously). I am fairly well versed in how the monoflo forced hot water up into the basebaord via monoflow T's and the pressure created from the circ. But would the setup I described ever work for a series loop coming off of the 1-1/4" main? Does anyone have any familiarity with a series loop / monoflow loop mix for two different zones. Does the setup I described sound like there are flaws? Or once we cut into the loop to add the additional baseboards and purge all the air out, will this work? I am sure I am not giving enough information so please let me know!