Hello; I have a Saito 30 and 2 OS 26's, I have a HP 29 as well but I doubt if that is one of your candidates. The OS' run well and are good on fuel, and are very quiet, not particularly fuel fussy. I have flown them down to -20 degrees and on our hottest days. The Saito seem a little more powerful, as I have one in a biplane and it powers it very well. Either of these engines would work just fine for you I'm sure. I have no complaints with either engine. I have no experience with the thunder tiger or ASP little 4-strokes, no one around here has seen one or heard one run. When I buy an engine I figure on keeping it forewver, so the purchase price is not as important as the quality of manufacture and parts back-up. Both engines will idle right down and purr along happily. The HP 29 I have has a sleeve valve and no rockers, It idles over 2500rpm and even though it runs well it is heavier and won't idle down low enough that the plane will sit on the runway without moving. Many other flyers at our field use the Os 26 but I haven't seen one of those new OS 30's yet, so I can't comment on that one either. I checked out OS' web page and they have a 26 surpass that they sell to the car guys that turns 22,000 rp
m, I can't imagine that but, would like tyo see that.
I would prefer the Saito myself, as I think it looks better, and I have lots of experience with other Saito engines and have come to trust them.