welcome to TBN, as i am writing you this note, i am applying your suggestion, just about the placing of the heaters, to my 425. the reason why i think, it will work for mine too is that in inclement weather we have the ability to disengage the hydraulics on our hydrostatic walkbehind mowers, which makes starting a snap.
here is what i did, this is based on your input and the experience with the hydrostatic walkbehind mowers:
one magnetic heater (Kat's Heaters, Handi-Heat Magnetic Heater, 200 Watts, Made by Five Star Mfg.Co, sorry no web link, the ones i found are all dead) by the hydraulic filter and another in the aft, on the hydraulic lines to and from the engine.
i know this is not engineering talk but in a few minutes, i hope your suggestion will worke for me too.