The Honda gx25 has a 52mm clutch. The thatsdax.com friction drive kit won't work for you because their kits are for 76mm clutches only.
Staton.com does offer a friction drive kit that you can use a Honda gx25 with. When you order a kit from him, you specify the size clutch drum you want as well as the roller size. For a Honda gx25 you will probably want a 7/8" roller - unless you only intend to ride on flat ground and/or you are very light - in which case you might be able to use a 1" roller.
Staton makes excellent products, but his instructions are a bit sparse and leave a lot of details up to the installer. Staton is very willing to give you advice as to what you need, but I would suggest calling rather than emailing as he doesn't get around to answering emails very well.
The Honda gx25 is only rated at 1.0 hp. It is a very usable, quiet and reliable engine for a motorized bicycle, but its power is on the low side.