Looking at the options and looking just at engine power (not factoring in weight or other dimensions):
P-61 | RR Dart Conversion based upon Cavalier Mustang | S-2T based conversion | Conair Firecat based conversion | Convair CV-580 style conversion | Basler BT-67 style conversion |
P&W R2800 Radial Piston | RR Dart Mk.510 (RDa.6) Turboprop | Honeywell TPE-331-15AW Turboprop | PT6A-67AF Turboprop | Allison 501/T56 Turboprop | PT6A-67R Turboprop |
2000hp - 2800hp ea depending upon model fitted | 1535 - 1,670 shp ea | 1,645 shp ea | 1,220 shp ea | 3,750 shp ea | 1,281 shp ea |
From this quick analysis one can see that the power rating for most turboprop versions is much less than that for the original R2800s, though this is offset significantly by the reduced weight, with all turboprops listed being lighter than the R2800s, some significantly so such as the PT6 and TPE331 which come in at around 15 - 20% of the weight of the radial.
That said, given the powers/weights and moreso, for modelling purposes, the availability of kits/conversions, maybe using the Allison 501/T56 as the basis might be the way to go. As indicated above, you could use the Convair CV-580 as the basis for this conversion. This started off as the Convair CV-340 (see below) with two R-2800 CB-17 engines each rated at 2,500 hp (1,890 kW) and with three-blade propellers, thus giving commonality with the original P-61.
The CV-580 added (amongst other changes) two Allison 501 D13D/H turboprop engines with four-blade propellers, each rated at 3,750 eshp (2,800 kW).
Therefore, the conversions are very comparable. Most importantly, from the model building perspective, you should be able to get parts by Lone Star Models - see
https://www.lonestarmodels.com/store/p_109399/lsm-40469-p-3-cowlings-props:Not only is this realistic based upon the real world, it is doable from a modelling perspective (though with still quite a bit of work to blend the engines in, especially given the P-61 has the twin booms) and also will look really different since I doubt anyone has done a T56 conversion of a WWII aircraft before.