JS0 cam in an 88/dynamo 500

Sounds very close to SS cam as used on all 500s after about 1963.  The journal size shouldn't be a problem. The problem with the smaller journal was allegedly in the 600 cc engines because the big end journals then did not overlap with the mains.  So the crank web had more bending stress.
Whether you would benefit is another issue.  Maybe someone can confirm but I suspect you would only notice it at 5000 to 7000 rpm...

Last 88 was 1962 or 63.  Then 88SS only for 64, 65 and 66.
I think the 88ss cam had 0.33" lift.  According to Dave Comeau website.

These engines are getting on a bit. A lumpier cam will give a bit more power at full throttle at higher revs. However how often are you going to be full throttle/max revs? Never hopefully.
The standard cam was developed for the 'normal' rider, whoever that was, giving good pulling power at medium revs and a decent tickover.
For my older machines I try not to cruise much above 4,000 rpm and have an self imposed red line at 5,000 rpm. 
HIstorical note: My 99 broke a rod when cruising along at 4,000 rpm on my way to my first day at a new job. Of course. 

There were other modifications to allow more performance.  Lighter and stiffer tubular alloy pushrods which need bigger pushrod holes in the barrel.  Shorter barrel to increase compression ratio.  88SS was perhaps the quickest 500cc bike on the road in 1962, but not now.

Hello Pete - get an 88SS crankshaft if you're thinking of roadburning.  If you are so doing I'd get a 10-ball main bearing on the timing side as well.   Later rods will also be a recommendation unless you know that they have been replaced with younger rods since it was built.   A six speed oil pump gear set may be worthwhile.  88SS pistons are 9,45 : 1 compression ratio.  You don't mention carbs on your downdraught head but I presume they will be paired 1 1/16 th inch monoblocs.  Don't fit larger than that otherwise you're just wasting fuel...Try to keep it below 8.5K.   Good luck, Howard  

The Norton twin engines all benefitted from the the flat foot camshaft followers fitted from 1959 onwards.  These being standard kit for the Q.R. and later SS camshaft engines.
The 99 engines had crankshafts with larger oilways which weakened the strength of the journals.
Poor machining of the journal radii added to the problem and broken crankshafts tended to  occur when max performance was demanded by riders with twitchy right hands.

My records indicate that the 88SS barrels kept the spigots and 3 start oil pumps until the end of production. The only noticeable alteration to the barrel being the front face of the pushrod tunnels moving forward around 2.5mm so that the insides could be widened to take the fatter SS pushrods.