I stuck a caravan electric heater in my mk1 van , and didn't want water cores in the cab. I pulled the unit apart and and fitted the heater elements and fuses etc. Also ditched the fan in favor of a Gixxer 750 radiator thermo fan, nice and flat n quiet except it sends the dash vents out the back doors on full speed !!
As for replacement cores, I was looking into Sierra ones, but have used TF Cortina ones as they are wider and crank heaps more heat than escort ones, highly recommend them , only slight mods to the heater box and your good to go..