Which engine for a vintage build?

Hey,

I'm thinking about getting a different motor for my vintage-style motorized bike (so the engine should be in this style too), but I just can't decide which one is the best choise. My chinagirl-50cc-thingy is just... horrible. I'm just a student so the budged isn't huge... In addition I can't weld or know anyone who would weld for free, so the motor mount is another point. As I'm from germany, it's hard to find an engine fitting the vintage style of my bike, but I found some:

Sachs "Stamo", different versions, but I don't want to go over 100cc in my frame:

http://kleinanzeigen.ebay.de/anzeig...-stamo-50-motor/100592916-222-2661?ref=search
http://kleinanzeigen.ebay.de/anzeig...r-160cm3-3-3,5ps/157746842-250-418?ref=search

The first one is 50cc, the second one 160cc. As far as i know, there is no clutch on those since they are stationary engines. Is adding a clutch possible somehow?

Sachs 98

http://kleinanzeigen.ebay.de/anzeig...htel-sachs-98er/112660181-222-3884?ref=search

Originally used in 1940-50 motorcycles; this one is one of the cheapest 98cc Sachs available... It got a clutch, two gears and got the best "vintage" look for my projekt, but spare parts are very expensive and hard to get

Motobecane

http://kleinanzeigen.ebay.de/anzeig...ane-typ-av7-dcl/148591498-218-1530?ref=search

50cc used in old french mopeds, design also fits the topic and got a centrifugal clutch, but as at the Sachs 98: hard to get, few spare parts, and so on.

Mower Motors

http://kleinanzeigen.ebay.de/anzeig...-briggs-straton/155236605-239-7714?ref=search

The one above is just an example, there are tons of those motors on german ebay... Cheap, but I dont really know mutch about them... I don't like this plastic at all, can you remove it? How do those engines look without those plastic things? I saw a few motorized bikes with these motors on the web, so probably somebody can tell me some pros and cons?

Simson Engine

http://www.akf-shop.de/images/produkte/i21/21877-10016839-02.jpg

Got one of these from one of my old GDR-mopeds, 50cc, reliable, 4 gears, manual clutch, tons of cheap spare and tuning parts, mine is 32 years old; just have to repair the ignition. The design is not the ideal one, all motor mounts are in the back, and the chain is on the right, not left...

What do you think? Which engine should I choose? I really can't decide.

Jonas from Berlin

Well, today my China-Engine killed himself, maybe he heard of my plans to replace him... Piston is broken. At the moment there are no low-priced Sachs 98 available, so I think i'm going for the Sachs Stamo or an Ilo stationary engine, found some in good condition with a clutch! As soon as I get the engine, I'll post some pictures and my progress in the vintage/BTR section Thanks for the support

Yeah I looked for those already but there were no villiers in germany