51 is in the 3 cu.in. class, making the 350/351 the nearest current model. 346xp also is in this power range, but is a little more expensive: it's more of a small hot rod saw for high speed work in small wood.
51 would handle 20" bar esp. if you went with narrow kerf .325 chain. Not optimum, but Homelite and Poulan have been getting by with it on even less powerful saws.
