Rizzer's home made picture looks awesome! If you can fab up something like that you'll have a bike that'd be competitive in the lower levels of modern events at the very least.
You could definitely make that fun to ride. For specs just search online for dimensions etc of any modern trials bike - Gasgas, Ossa, Sherco, Scorpa, Montesa etc.
I wouldn't worry too much about a cradle frame (I don't think any modern bikes use full cradle frames - I know my Montesa doesn't). The engine really isn't the big concern, just get something light. Focus on the chassis.
You really need an 18" rear wheel as that's the only size trials tires come in and they are rather unique. A 21" front would be good too, but a proper trials front is less critical. A knobby front would work fine for most stuff.
Regarding the two stroke engines and exhausts they are tuned to run well at very low rpm and be very controllable. They aren't especially powerful. I think my liquid cooled, 250cc two stroke makes about 17hp. IIRC it actually has a rather short stroke as compared to my CR250R motocross bike. I'm not sure why this is,but it works out ok. It may simply be to keep the engine dimensions smaller.
The exhaust is very critical on a two stroke to how the engine runs. There's a whole literature on exhaust tuning and design - you'd want to read up before building a system I think. I don't have a quarter the knowledge to design my own expansion chamber. I will say that on a trials bike exhaust design looks to be driven mostly by packaging requirements and only secondarily by performance requirements.
Engine performance really isn't a big deal. You need very little power. What you need is a really compact bike, light weight bike with a controllable engine and super-low gearing. A 100cc-ish two stroke would be fine. Folks do all sorts of amazing stuff on 125 and even 80cc trials bikes with no issues - just run a bit lower gearing and rev the motor more.
I've known folks to take bikes apart and put them in their airplane luggage. Would that be illegal for your country even if you won't be registering the bike for road use? Here you can realistically do most anything you want as long as its only used off road "closed course" only. In '97 when the honda CR250R was available in Japan but not the US the editors of Dirt Rider magazine flew to Japan, bought a CR250R, took it apart in the parking lot and brought it home on the plane in their luggage. They published the whole story with tons of pictures - good fun
I have an '01 Montesa Cota 315R and it works well enough for me. They are about 1k pounds in the UK.
lamotovita, D6 as in AMA District 6. I am moving up next event (I've only been at this for a couple of months so started out only one class up from the bottom - cleaned the whole day). I certainly couldn't clean a novice D6 section on a non-trials bike, but I couldn't even ride around the section with a whole bunch of dabs. I'd probably have to get off the bike and drag it around corners or make a 30-point turn or similar. Many sections are pretty darn tight. A few are more straight and wouldn't be hard on a big bike and one was really straight and rocky and would have probably been easier on a big bike
Maybe your novice is more like our vintage-novice - that I could defo ride on a dirt bike as its a much less tight course - or maybe I just suck.
Anyway, I totally agree that you can have a ton of fun doing technical, trials-ish stuff on a dirt bike.
Steep ledges (this was part of an old trials section)