So your looking for a side valve J van engine, head over to flea-bay home of con men and time wasters there are FOUR for sale.
Link to the item,http://www.ebay.co.uk/itm/Morris-J-type-van-side-valve-engine-/251447743603?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item3a8b741473#ht_279wt_958
Another J engine, slightly more complete and maybe a gold seal unit.
Link to the item, http://www.ebay.co.uk/itm/MORRIS-OXFORD-MO-J-TYPE-VAN-INLINE-4-SIDE-VALVE-ENGINE-1948-1953/181329723574?rt=nc&_trksid=p2047675.m1851&_trkparms=aid%3D222002%26algo%3DSIC.FIT%26ao%3D1%26asc%3D286%26meid%3D5058631301947579773%26pid%3D100005%26prg%3D1048%26rk%3D2%26rkt%3D5%26sd%3D251447743603%26
Yet another engine, this one still seems to be fitted in an old MO car,
Link here,http://www.ebay.co.uk/itm/MO-Morris-Oxford-Engine-1476cc-Side-Valve-Straight-4-complete-and-running-sweet/141192853344?rt=nc&_trksid=p2047675.m1851&_trkparms=aid%3D222002%26algo%3DSIC.FIT%26ao%3D1%26asc%3D286%26meid%3D5058734250171627809%26pid%3D100005%26prg%3D1048%26rk%3D1%26rkt%3D5%26sd%3D181329723574%26
And yet another engine,
Link here, http://www.ebay.co.uk/itm/Vintage-MO-Morris-Oxford-Engine-1476cc-Side-Valve-Straight-4-circa-1948-53/231145246822?rt=nc&_trksid=p2047675.m1851&_trkparms=aid%3D222002%26algo%3DSIC.FIT%26ao%3D1%26asc%3D286%26meid%3D5058750209056873612%26pid%3D100005%26prg%3D1048%26rk%3D4%26rkt%3D5%26sd%3D141192853344%26
Prices, start at £215 up to a high £600, all of these will need at least a good strip down and check over.