It's good news for you. SVA is merely a super MOT. Depending on Low Power Moped/Moped/Motorcycle classification, you have to have various requirements fulfilled. 50cc, 1kw/1.3 horse and 16mph flat out on a vehicle capable of being pedalled is the minimalist way to go. If you went trike or quadricycle moped qualified, then the intriguing option of 4kw non-spark ignition engine is available. Nothing in the rules prevents hot bulb ignition petrol engines, if you don't want a diesel or steam engine.
I have 2 slow projects on the go. One is a bicycle with a minimoto engine over the back wheel, that may go through SVA, I don't know. The other is the creation, using pre-existing parts and documents, of a Genuine Vintage Fake, to be titled the Gravis Et Lento.
If you haven't already looked, there's a section full of pictures, with all sorts of ideas and solutions. http://motorbicycling.com/forumdisplay.php?f=15
Look up also the works of Cannonball2 and his 90 degree direction changing belt drive. An engine may be bulky one way, but narrow if you sidestep convention.