My favorite engine to build on the 3R case (2.2T) is the 2.45 Mod S (Twin plug if the budget allows.) Basically a detuned long stroke 2.5 ST engine.
86 mm is about as large as the 3R case can hold without boring the spigot and weakening the case. Plus the 86 mm Biral barrels are very affordable.
The engine is a 2.45 liter with DC Mod “S” cam and 9.5:1 compression.
Balanced the entire engine to 1gram.
CaseSavers, oil by-pass mod, 3.2 Carrera pump, line bore and oil squirters.
The 2.4 (70.4 mm) crank is magged, polished and cross drilled.
The pistons are JE 9.5:1 forged racing pistons in new 86mm Biral barrels.
The rods rebuilt new bolts & 12 point flanged nut.
The heads are 2.2 T with 36mm intake ports and machined for twin plugs.
Phosphorus /bronze valve guides and new valves (47, 41) installed.
The Supertec 240 valve springs. The head studs are of course Supertec.
40 mm Webers, SSI exhaust and M&K muffler.
This engine will produce 180 RWHP at 6400 with more to go if needed.
BTW: More torque then any RS spec carbureted engine.