Despite the miss-information being posted; On stroker builds, since the cylinder sits higher by almost 5.33MM, it does make installing or removing the spark Plug much harder on an 85-86 ATC250R...this is Not the case for TRX250R 86-89. Also keep in mind aftermarket cylinders use taller heads with a 'straight' spark plug, and not angled to the Left away from the Top Frame rail as Honda did on the OEM heads.
Their is a reason why Honda angled the spark plugs, and it had nothing to do with performance and everything to do with ease of spark plug installation & removal on the ATC250R. They kept the angled spark plug throughout the TRX250R models because of manufacturing convenience.
As you will see; with an aftermarket cylinder & head, plus a stroker crankshaft & spacer plate or taller cylinder (with built in spacer plate as matuus stated), the spark plug can be a pain to fully access easily. Their are 'tricks' that can be done, however; I will leave those tricks for your builder to detail you on.