guilderoy,
I have been using A-90HP's for a few years and am very happy with them. They are well made and very powerful, especially with the right mods and pipe. Recently I have been using one in a rigger that I have had on radar at 80mph with very little development.
In Australia I have been getting them from Craig Chenco, and have not needed many parts, but he has been able to get them.
However they are probably the most complicated motors I have had to work on. They have a very unique design, which is very strong and performs well, but the downside of this is they are more complicated than many motors. I dont know how much experience you have, but there may be simpler choices if you are relatively inexperienced.
Ian.