I believe 35DL at first used the A98 aluminium engine fitted with conventional points ignition....later versions after 1981? were fitted with the first type of electronic ignition...there were changes to the block casting so it has to be replacement part like for like I suspect.
I think unless things have changed very recently that both ignition types parts are still available...points, coil and electronic unit. http://www.midlandpower.co.uk/spares/se ... artType/05
This being one link to a spares provider to the mower trade.
If you can't find new then suspect it will be down to looking out for s/h item...they were made in massive numbers so will be about...I think the first electronic ignition type continued right through to perhaps early 1990s when the second type came out...parts for them were not interchangeable with the first type.
Do check that the fault actually lies with the unit...as the shock I suspect will be in the price..??!!
Early fitments suffered from wire chafing...so do look out for damaged insulation on wiring
..and check that all is well with the stop wire that goes back and is held from earth near the carb..that it is not damaged and losing the spark.
Also the suppressor cap that joins HT lead to plug top can fail....and if it has been removed many times in past history the wire in the lead end can break up leaving the cap only screwed into insulation.!!
Never dismiss plug failure...as they can go on for years but equally well fail.
All the best,