Your .61 HB should accept most O.S. glow plugs just fine. The #8 and A3 are quite popular for 2-strokes, but you might want to use a colder plug if you're using high nitro content fuel. Higher nitro fuel often calls for a colder plug while lower nitro/no nitro calls for a hotter plug.

Generally speaking, most 2-stroke aircraft engines will run fine with a Hot (A3) or Med Hot (#8) plug with 15% or lower nitro. If you're running 30% nitro helicopter fuel in your 2-stroke engine (waste of money), you'd probably want to go with a Med or Med cool plug.

Hope this makes sense!