In SE PA, unless your house is a leaky uninsulated wreck with cracked single-pane windows I'd be shocked if your design day heat load was even 50,000BTUs on a 900' house. Using a crusty old-schooler's "Lessee, OK condition single panes, not so insulated walls, call it 25BTU/foot, 35BTU just in case" crude reckoning you'd come up with a heat load of 32K, and even that's likely to oversized by 2x if the place has some insulation and storm windows or double-panes.
In any sort of reasonable shape I suspect you're looking at 25KBTU/hr tops. That would make even the smallest oil-boilers more than 2x oversized, but there are some tiny cast-iron LP units in the 30-35KBTU range, and some modulating condensing versions that go even lower.
Big oversized high-mass radiators can be pretty good for getting better than 90%+ efficiency out of a modulating-condensing propane burner. (which would usually be side vented with plastic flue.) Your 160KBTU oil boiler in that small a house was probably only getting 55-60% efficiency out of the oil due to oversizing & age so a mod-con would be a reduction in annual heating cost. An 86% efficiency side-vented cast iron LP boiler can do OK if it's RIGHT SIZED for the actual heating load. IIRC you'd have to drop to ~82% efficiency to be able to run it up the flue, but the flue would be oversized for any right-sized boiler and would need a narrowing liner, so there's no incentive to go lower-efficiency/higher-cost.
A smallest best-in-class triple-pass oil boiler would cost about the same to run as an LP mod-con, but that too would require adjustments to the exhaust venting from your 5x oversized pig of a boiler.
It may not work for you (from a floor-plan or aesthetics perspective) but a 2 or 2.5 ton (24-30KBTU) ductless mini-split heat pump would likely cost less up front and cost less to run than any oil or LP burner, and would likely cover 100% of your heat load. A 2-head "multi-split" with one interior unit for each floor would still be cheaper than a mod-con or high-efficiency oil boiler, and at 15cent/kwh electricity would 40% cheaper to run than 90% LP or 87% oil at current prices. (And you'd get high-efficiency air conditioning out of it too.) Where you're off the gas-grid, ductless split systems tend to be the cheapest/best way to go, as long as it meets the design day heating requirements.
Design temps in SE PA are in the low-teens F, which isn't a probably for any mini-split. Most still work at +4F, some are still able to put out 70%+ of rated heat at -13F outdoor temps. Any of them on this list will work at 0F, most are good for colder. Anything with an HSPF of 8.5 or greater will be pretty cheap to run.